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The  International  Scieiitjfe :  3«?r  es  ; 

The    New    Physics 

and  its   Evolution 


By 

Lucien    Poincare 

>/ 

Inspecteur-G6n6ral  de  1  Instruction  Publique 


Being  the  authorised  translation  of 
La  Physique  Moderne,  son  evolution " 


'.r-      - 

OF  THE 


NEW    YORK 
D.  APPLETON   AND   COMPANY 

1908 


PHYSIOS  DEPT. 


Prefatory  Note 


M.  LUCIEN  POINCARE  is  one  of  the  distinguished 
family  of  mathematicians  which  has  during  the  last 
few  years  given  a  Minister  of  Finance  to  the  Republic 
and  a  President  to  the  Academie  des  Sciences.  He 
is  also  one  of  the  nineteen  Inspectors-General  of 
Public  Instruction  who  are  charged  with  the  duty  of 
visiting  the  different  universities  and  lycfas  in  France 
and  of  reporting  upon  the  state  of  the  studies  there 
pursued.  Hence  he  is  in  an  excellent  position  to 
appreciate  at  its  proper  value  the  extraordinary 
change  which  has  lately  revolutionized  physical 
science,  while  his  official  position  has  kept  him  aloof 
from  the  controversies  aroused  by  the  discovery  of 
radium  and  by  recent  speculations  on  the  constitution 
of  matter. 

M.  Poincare's  object  and  method  in  writing  the 
book  are  sufficiently  explained  in  the  preface  which 
follows ;  but  it  may  be  remarked  that  the  best  of 
methods  has  its  defects,  and  the  excessive  condensa- 
tion which  has  alone  made  it  possible  to  include  the 

210946 


vi  PREFATORY  NOTE 

last  decade's  discoveries  in  physical  science  within 
a  compass  of  some  300  pages  has,  perhaps,  made  the 
facts  here  noted  assimilable  with  difficulty  by  the 
untrained  reader.  To  remedy  this  as  far  as  possible, 
I  have  prefixed  to  the  present  translation  a  table  of 
contents  so  extended  as  to  form  a  fairly  complete 
digest  of  the  book,  while  full  indexes  of  authors  and 
subjects  have  also  been  added.  The  few  notes 
necessary  either  for  better  elucidation  of  the  terms 
employed,  or  for  giving  account  of  discoveries  made 
while  these  pages  were  passing  through  the  press, 
may  be  distinguished  from  the  author's  own  by  the 
signature  "  ED.  " 


THE  EDITOR. 


ROYAL  INSTITUTION  OF  GREAT  BRITAIN, 
April  1907. 


Author's    Preface 

DURING  the  last  ten  years  so  many  works  have  ac- 
cumulated in  the  domain  of  Physics,  and  so  many 
new  theories  have  been  propounded,  that  those  who 
follow  with  interest  the  progress  of  science,  and 
even  some  professed  scholars,  absorbed  as  they  are 
in  their  own  special  studies,  find  themselves  at  sea 
in  a  confusion  more  apparent  than  real. 

It  has  therefore  occurred  to  me  that  it  might  be 
useful  to  write  a  book  which,  while  avoiding  too 
great  insistence  on  purely  technical  details,  should 
try  to  make  known  the  general  results  at  which 
physicists  have  lately  arrived,  and  to  indicate  the 
direction  and  import  which  should  be  ascribed  to 
those  speculations  on  the  constitution  of  matter,  and 
the  discussions  on  the  nature  of  first  principles, 
to  which  it  has  become,  so  to  speak,  the  fashion 
of  the  present  day  to  devote  oneself. 

I  have  endeavoured  throughout  to  rely  only  on 
the  experiments  in  which  we  can  place  the  most 
confidence,  and,  above  all,  to  show  how  the  ideas 


viii  PREFACE 

prevailing  at  the  present  day  have  been  formed,  by 
tracing  their  evolution,  and  rapidly  examining  the 
successive  transformations  which  have  brought  them 
to  their  present  condition. 

In  order  to  understand  the  text,  the  reader  will 
have  no  need  to  consult  any  treatise  on  physics, 
for  I  have  throughout  given  the  necessary  definitions 
and  set  forth  the  fundamental  facts.  Moreover,  while 
strictly  employing  exact  expressions,  I  have  avoided 
the  use  of  mathematical  language.  Algebra  is  an 
admirable  tongue,  but  there  are  many  occasions 
where  it  can  only  be  used  with  much  discretion. 

Nothing  would  be  easier  than  to  point  out  many 
great  omissions  from  this  little  volume;  but  some, 
at  all  events,  are  not  involuntary. 

Certain  questions  which  are  still  too  confused  have 
been  put  on  one  side,  as  have  a  few  others  which 
form  an  important  collection  for  a  special  study 
to  be  possibly  made  later.  Thus,  as  regards  elec- 
trical phenomena,  the  relations  between  electricity 
and  optics,  as  also  the  theories  of  ionization,  the 
electronic  hypothesis,  etc.,  have  been  treated  at  some 
length ;  but  it  has  not  been  thought  necessary  to 
dilate  upon  the  modes  of  production  and  utilization 
of  the  current,  upon  the  phenomena  of  magnetism, 
or  upon  all  the  applications  which  belong  to  the 
domain  of  Electro  technics. 

L.  POINCARE. 


Contents 


PAGES 

EDITOR'S  PREFATORY  NOTE    -   .         .         .         .         .  v 

AUTHOR'S  PREFACE vii 

TABLE  OF  CONTENTS          .......         ix 

CHAPTER   I 
THE  EVOLUTION  OF  PHYSICS 1-18 

Revolutionary  change  in  modern  Physics  only  apparent : 
evolution  not  revolution  the  rule  in  Physical  Theory — 
Revival  of  metaphysical  speculation  and  influence  of 
Descartes :  all  phenomena  reduced  to  matter  and  move- 
ment— Modern  physicists  challenge  this :  physical,  un- 
like mechanical,  phenomena  seldom  reversible — Two 
schools,  one  considering  experimental  laws  imperative, 
the  other  merely  studying  relations  of  magnitudes :  both 
teach  something  of  truth — Third  or  eclectic  school — 
Is  mechanics  a  branch  of  electrical  science  ? 

CHAPTER  II 
MEASUREMENTS  ....  ....  19-50 

§  1.  Metrology :  Lord  Kelvin's  view  of  its  necessity — Its 
definition  (pp.  19-21).  §2.  The  Measure  of  Length: 
Necessity  for  unit— Absolute  length— History  of  Stand- 
ard— Description  of  Standard  Metre — Unit  of  wave- 
lengths preferable  —  The  International  Metre  (pp. 
21-30).  §  3.  The  Measure  of  Mass :  Distinction  be- 
tween mass  and  weight — Objections  to  legal  kilo- 
gramme and  its  precision — Possible  improvement  (pp. 
31-33).  §  4.  The  Measure  of  Time :  Unit  of  time  the 
second — Alternative  units  proposed — Improvements  in 
chronometry  and  invar  (pp.  34-37).  §  5.  The  Measure 


x  CONTENTS 

PAGES 

of  Temperature :  Fundamental  and  derived  units — Or- 
dinary unit  of  temperature  purely  arbitrary — Absolute 
unit  mass  of  H  at  pressure  of  1  m.  of  Hg  at  0°  C. — 
Divergence  of  thermometric  and  thermodynamic  scales 
— Helium  thermometer  for  low,  thermo-electric  couple 
for  high,  temperatures — Lummer  and  Pringsheim's  im- 
provements in  thermometry  (pp.  37-42).  §  6.  Derived 
Units  and  Measure  of  Energy :  Importance  of  erg  as 
unit  —  Calorimeter  usual  means  of  determination  — 
Photometric  units  (pp.  42-45).  §  7.  Measure  of  Phy- 
sical Constants :  Constant  of  gravitation — Discoveries  of 
Cavendish,  Vernon  Boys,  Eb'tvos,  Richarz  and  Krigar- 
Menzel  —  Michelson's  improvements  on  Fizeau  and 
Foucault's  experiments — Measure  of  speed  of  light  (pp. 
45-50). 

CHAPTER  III 
PRINCIPLES         ........          51-104 

§  1.  The  Principles  of  Physics :  The  Principles  of 
Mechanics  affected  by  recent  discoveries — Is  mass 
indestructible  ?  —  Landolt  and  Heydweiller's  experi- 
ments— Lavoisier's  law  only  approximately  true — 
Curie's  principle  of  symmetry  (pp.  51-55).  §  2.  The 
Principle  of  the  Conservation  of  Energy :  Its  evolution  : 
Bernoulli,  Lavoisier  and  Laplace,  Young,  Rumford. 
Davy,  Sadi  Carnot,  and  Robert  Mayer — Mayer's  draw- 
backs— Error  of  those  who  would  make  mechanics  part 
of  energetics — Verdet's  predictions — Rankine  inventor 
of  energetics— Usefulness  of  Work  as  standard  form  of 
energy — Physicists  who  think  matter  form  of  energy — 
Objections  to  this — Philosophical  value  of  conservation 
doctrine  (pp.  55-72).  §  3.  The  Principle  of  Carnot  and 
Clausius :  Originality  of  Carnot's  principle  that  fall  of 
temperature  necessary  for  production  of  work  by  heat — 
Clausius'  postulate  that  heat  cannot  pass  from  cold  to 
hot  body  without  accessory  phenomena — Entropy  result 
of  this — Definition  of  entropy — Entropy  tends  to  in- 
crease incessantly — A  magnitude  which  measures  evolu- 
tion of  system — Clausius'  and  Kelvin's  deduction  that 
heat  end  of  all  energy  in  Universe — Objection  to  this — 
Carnot's  principle  not  necessarily  referable  to  mechanics 
— Brownian  movements  —  Lippinann's  objection  to 
kinetic  hypothesis  (pp.  72-87).  §4.  Thermodynamics: 
Historical  work  of  Massieu,  Willard  Gibbs,  Helmholtz, 
and  Duhern — Willard  Gibbs  founder  of  thermodynamic 
statics,  Van  t'Hoff  its  reviver — The  Phase  Law — Raveau 
explains  it  without  thermodynamics  (pp.  87-92).  §  5. 
Atomism:  Connection  of  subject  with  preceding — 


CONTENTS  xi 

PAGES 

Hanuequin's  essay  on  the  atomic  hypothesis — Molecular 
physics  in  disfavour — Surface-tension,  etc.,  vanishes 
when  molecule  reached — Size  of  molecule  —  Kinetic 
theory  of  gases — Willajd  Gibbs  and  Boltzmann  introduce 
into  it  law  of  probabilities — Mean  free  path  of  gaseous 
molecules  —  Application  to  optics  —  Final  division  of 
matter  (pp.  87-104): 

CHAPTER  IV 
THE  VARIOUS  STATES  OF  MATTER     ....        105-142 

§  1.  The  Statics  of  Fluids:  Researches  of  Andrews, 
Cailletet,  and  others  on  liquid  and  gaseous  states — 
Amagat's  experiments — Van  der  Waals'  equation — Dis- 
covery of  corresponding  states — Amagat's  superposed 
diagrams — Exceptions  to  law — Statics  of  mixed  fluids — 
Kamerlingh  Onnes'  researches — Critical  Constants — 
Characteristic  equation  of  fluid  not  yet  ascertainable 
(pp.  105-117).  §  2.  The  Liquefaction  of  Gases  and 
Low  Temperatures:  Linde's,  Siemens',  and  Claude's 
methods  of  liquefying  gases— Apparatus  of  Claude  de- 
scribed— De war's  experiments — Modification  of  electrical 
properties  of  matter  by  extreme  cold :  of  magnetic  and 
chemical — Vitality  of  bacteria  unaltered — Ramsay's  dis- 
covery of  rare  gases  of  atmosphere— Their  distribution 
in  nature — Liquid  hydrogen — Helium  (pp.  117-126). 
§  3.  Solids  and  Liquids :  Continuity  of  Solid  and  Liquid 
States— Viscosity  common  to  both — Also  Rigidity — 
Spring's  analogies  of  solids  and  liquids — Crystallization 
— Lehmann's  liquid  crystals — Their  existence  doubted 
— Tamman's  view  of  discontinuity  between  crystalline 
and  liquid  states  (pp.  126-134).  §  4.  The  Deformation 
of  Solids  :  Elasticity  — Hoocke's,  Bach's,  and"  Bouasse's 
researches — Voigt  on  the  elasticity  of  crystals — Elastic 
and  permanent  deformations—  Brilloum's  states  of  un- 
stable equilibria  —  Duheni  and  the  therrnodyuamic 
postulates  —  Experimental  confirmation  —  Guillaume's 
researches  on  nickel  steel — Alloys  (pp.  135-142). 


CHAPTER  V 
SOLUTIONS  AND  ELECTROLYTIC  DISSOCIATION    .        .        143-168 

§  1.  Solution:  Kirchhoifs,  Gibb's,  Duhem's  and  Van 
t'Hoffs  researches  (pp.  143-146).  §2.  Osmosis:  His- 
tory of  phenomenon— Traube  and  biologists  establish 
existence  of  semi-permeable  walls — Villard's  experiments 


xii  CONTENTS 

PAGES 

with  gases — Pfeffer  shows  osmotic  pressure  proportional 
to  concentration— Disagreement  as  to  cause  of  pheno- 
menon (pp.  146-151).  §  3.  Osmosis  applied  to  Solution  : 
Van  t'Hotf's  discoveries — Analogy  between  dissolved 
body  and  perfect  gas — Faults  in  analogy  (pp.  151-155). 
§  4.  Electrolytic  Dissociation  :  Van  t'Hoff's  and 
Arrhenius'  researches  —  Ionic  hypothesis  of  —  Fierce 
opposition  to  at  first — Arrhenius'  ideas  now  triumphant 
— Advantages  of  Arrhenius'  hypothesis — "The  ions 
which  react" — Ostwald's  conclusions  from  this — Nernst's 
theory  of  Electrolysis — Electrolysis  of  gases  makes  elec- 
tronic theory  probable — Faraday's  two  laws — Valency — 
Helmholtz's  consequences  from  Faraday's  laws  (pp. 
155-108). 

CHAPTER  -  VI 
THE  ETHER 169-207 

§  1.  The  Luminiferous  Ether:  First  idea  of  Ether  due 
to  Descartes — Ether  must  be  imponderable — Fresnel 
shows  light  vibrations  to  be  transverse — Transverse 
vibrations  cannot  exist  in  fluid— Ether  must  be  discon- 
tinuous (pp.  169-175).  §2.  Radiations:  Wave-lengths 
and  their  measurements — Rubens'  and  Lenard's  re- 
searches—Stationary waves  and  colour-photography  — 
Fresnel 's  hypothesis  opposed  by  Neumann — Wiener's 
and  Cotton's  experiments  (pp.  175-182).  §  3.  The 
Electromagnetic  Ether:  Ampere's  advocacy  of  mathe- 
matical expression — Faraday  first  shows  influence  of 
medium  in  electricity — Maxwell's  proof  that  light-waves 
electromagnetic — His  unintelligibility — Required  confir- 
mation of  theory  by  Hertz  (pp.  182-189).  §  4.  Electrical 
Oscillations :  Hertz's  experiments  —  Blondlot  proves 
electromagnetic  disturbance  propagated  with  speed  of 
light — Discovery  of  ether  waves  intermediate  between 
Hertzian  and  visible  ones — Rubens'  and  Nichols'  ex- 
periments— Hertzian  and  light  rays  contrasted — Pres- 
sure of  light  (pp.  189-194).  §5.  The  X-Rays: 
Rontgeu's  discovery — Properties  of  X-rays — Not  homo- 
geneous —  Rutherford  and  M'Clung's  experiments  on 
energy  corresponding  to  —  Barkla's  experiments  on 
polarisation  of — Their  speed  that  of  light — Are  they 
merely  ultra-violet?— Stokes  and  Wiechert's  theory  of 
independent  pulsations  generally  preferred  —  J.  J. 
Thomson's  idea  of  their  formation — Sutherland's  and  Le 
Bon's  theories — The  N-Rays  -Blondlot's  discovery — 
Experiments  cannot  be  repeated  outside  France— Gutton 
and  Mascart's  confirmation  —  Negative  experiments 
prove  nothing — Supposed  wave-length  of  N-rays  (pp. 


CONTENTS  xiii 

PAGES 


194-202).  §  6.  The  Ether  and  Gravitation  :  Descartes' 
and  Newton's  ideas  on  gravitation — Its  speed  and  other 
extraordinary  characteristics  —  Lesage's  hypothesis  — 
Cremieux'  experiments  with  drops  of  liquids — Hypo- 
thesis of  ether  insufficient  (pp.  203-207). 


CHAPTEE  VII 
WIRELESS  TELEGRAPHY 208-234 

§  1.  Histories  of  wireless  telegraphy  already  written, 
and  difficulties  of  the  subject  (pp.  208-211),  §  2.  Two 
systems:  that  which  uses  the  material  media  (earth, 
air,  or  water),  and  that  which  employs  ether  only  (pp. 
211-213).  §  3.  Use  of  earth  as  return  wire  by  Steinheil 
— Morse's  experiments  with  water  of  canal — Seine  used  as 
return  wire  during  siege  of  Paris — Johnson  and  Melhuish's 
Indian  experiments — Preece's  telegraph  over  Bristol 
Channel— He  welcomes  Marconi  (pp.  213-218).  §  4. 
Early  attempts  at  transmission  of  messages  through 
ether  —  Experiments  of  Rathenau  and  others  (pp. 
218-220).  §  5.  Forerunners  of  ether  telegraphy  :  Clerk 
Maxwell  and  Hertz— Dolbear,  Hughes,  and  Graham 
Bell  (pp.  220-225).  §  6.  Telegraphy  by  Hertzian 
waves  first  suggested  by  Threlfall — Crookes',  Tesla's, 
Lodge's,  Rutherford's,  and  Popoffs  contributions — 
Marconi  first  makes  it  practicable  (pp.  225-228).  §  7. 
The  receiver  in  wireless  telegraphy— Varley's,  Calzecchi- 
Onesti's,  and  Branly's  researches  —  Explanation  of 
coherer  still  obscure  (pp.  228-230).  §  8.  Wireless 
telegraphy  enters  the  commercial  stage  —  Defect  of 
Marconi's  system  —  Braun's,  Armstrong's,  Lee  de 
Forest's,  and  Fessenden's  systems  make  use  of  earth — 
Hertz  and  Marconi  entitled  to  foremost  place  among 
discoverers  (pp.  230-234). 


CHAPTEK  VIII 
THE  CONDUCTIVITY  OF  GASES  AND  THE  IONS    .        .        235-257 

§  1.  The  Conductivity  of  Gases:  Relations  of  matter  to 
ether  cardinal  problem — Conductivity  of  gases  at  first 
misapprehended — Erman's  forgotten  researches— Giese 
first  notices  phenomenon — Experiment  with  X-rays — 
J.  J.  Thomson's  interpretation — Ionized  gas  not  obedi- 
ent to  Ohm's  law — Discharge  of  charged  conductors  by 
ionized  gas  (pp.  235-242).  §  2.  The  Condensation  of 
Water-vapour  by  Ions :  Vapour  will  not  condense  with- 


xiv  CONTENTS 

PAGES 

out  nucleus — Wilson's  experiments  on  electrical  con- 
densation— Wilson  and  Thomson's  counting  experiment 
— Twenty  million  ions  per  c.cm.  of  gas — Estimate  of 
charge  borne  by  ion — Speed  of  charges — Zeleny's  and 
Langevin's  experiments — Negative  ions  T7JW  °f  size  °f 
atoms — Natural  unit  of  electricity  or  electrons  (pp. 
242-249).  §  3.  How  Ions  are  Produced:  Various 
causes  of  ionization — Moreau's  experiments  with  alka- 
line salts— Barus  and  Bloch  on  ionization  by  phos- 
phorus vapours — Ionization  always  result  of  shock  (pp. 
249-253).  §  4.  Electrons  in  Metals:  Movement  of 
electrons  in  metals  foreshadowed  by  Weber — Giese's, 
Riecke's,  Drude's,  and  J.  J.  Thomson's  researches— Path 
of  ions  in  metals  and  conduction  of  heat — Theory  of 
Lorentz  —  Hesehus'  explanation  of  electrification  by 
contact  —  Emission  of  electrons  by  charged  body  — 
Thomson's  measurement  of  positive  ions  (pp.  253-257). 


CHAPTER   IX 
CATHODE  RAYS  AND  RADIOACTIVE  BODIES         .        .        258-292 

§1.  The  Cathode  Rays :  History  of  discovery — Crookes' 
theory  —  Lenard  rays  —  Perrin's  proof  of  negative 
charge — Cathode  rays  give  rise  to  X-rays — The  canal 
rays — Villard's  researches  and  magneto-cathode  rays — 
loiioplasty— Thomson's  measurements  of  speed  of  rays 
— All  atoms  can  be  dissociated  (pp.  258-268).  §  2. 
Radioactive  Substances :  Uranic  rays  of  Niepce  de  St 
Victor  and  Becquerel — General  radioactivity  of  matter — 
Le  Bon's  and  Rutherford's  comparison  of  uranic  with 
X  rays — Pierre  and  Mme.  Curie's  discovery  of  polonium 
and  radium — Their  characteristics — Debierne  discovers 
actinium  (pp.  269-274).  §  3.  Radiations  and  Emana- 
tions of  Radioactive  Bodies:  Giesel's,  Becquerel's,  and 
Rutherford's  Researches— a,  0,  and  7  rays— Sagnac's 
secondary  rays — Crookes'  spinthariscope — The  emanation 
— Ramsay  and  Soddy's  researches  upon  it— Transforma- 
tions of  radioactive  bodies — Their  order  (pp.  274-282). 
§  4.  Disaggregation  of  Matter  and  Atomic  Energy: 
Actual  transformations  of  matter  in  radioactive  bodies 
—Helium  or  lead  final  product— Ultimate  disappearance 
of  radium  from  earth — Energy  liberated  by  radium : 
its  amount  and  source — Suggested  models  of  radioactive 
atoms  —  Generalization  from  radioactive  phenomena 
— Le  Bon's  theories — Ballistic  hypothesis  generally 
admitted — Does  energy  come  from  without— Sagnac's 
experiments— Elster  and  Geitel's  contra  (pp.  282-292). 


CONTENTS  xv 

CHAPTER  X  PAGES 

THE  ETHER  AND  MATTER 293-321 

§  1.  The  Relations  between  the  Ether  and  Matter: 
Attempts  to  reduce  all  matter  to  forms  of  ether — Emis- 
sion and  absorption  phenomena  show  reciprocal  action — 
Laws  of  radiation — Radiation  of  gases — Production  of 
spectrum — Differences  between  light  and  sound  varia- 
tions show  difference  of  media — Cauchy's,  Briot's,  Car- 
vallo's  and  Boussinesq's  researches — Helmholtz's  and 
Poincare's  electromagnetic  theories  of  dispersion  (pp. 
293-302).  §2.  The  Theory  of  Lorentz :  Mechanics  fails 
to  explain  relations  between  ether  and  matter — Lorentz 
predicts  action  of  magnet  on  spectrum — Zeeman's  ex- 
periment —  Later  researches  upon  Zeeman  effect — 
Multiplicity  of  electrons  —  Lorentz's  explanation  of 
thermoelectric  phenomena  by  electrons— Maxwell's  and 
Lorentz's  theories  do  not  agree — Lorentz's  probably  more 
correct — Earth's  movement  in  relation  to  ether  (pp. 
302-311).  §3.  The  Mass  of  Electrons:  Thomson's  and 
Max  Abraham's  view  that  inertia  of  charged  body  due 
to  charge — Longitudinal  and  transversal  mass — Speed 
of  electrons  cannot  exceed  that  of  light  —  Ratio  of 
charge  to  mass  and  its  variation  —  Electron  simple 
electric  charge  —  Phenomena  produced  by  its  accelera- 
tion (pp.  311-316).  §  4.  New  Views  on  Ether  and 
Matter  :  Insufficiency  of  Larmor's  view — Ether  definable 
by  electric  and  magnetic  fields  —  Is  matter  all  elec- 
trons? Atom  probably  positive  centre  surrounded  by 
negative  electrons  —  Ignorance  concerning  positive 
particles — Successive  transformations  of  matter  probable 
— Gravitation  still  unaccounted  for  (pp.  316-321). 

CHAPTER  XI 
THE  FUTURE  OF  PHYSICS 322-328 

Persistence  of  ambition  to  discover  supreme  principle 
in  physics  —  Supremacy  of  electron  theory  at  present 
time — Doubtless  destined  to  disappear  like  others — 
Constant  progress  of  science  predicted — Immense  field 
open  before  it. 

INDEX  OF  NAMES 329-335 

INDEX  OF  SUBJECTS 336-344 


The  New   Physics  and 
its  Evolution 

CHAPTEK  I 
THE  EVOLUTION  OF  PHYSICS 

THE  now  numerous  public  which  tries  with  some 
success  to  keep  abreast  of  the  movement  in  science, 
from  seeing  its  mental  habits  every  day  upset,  and 
from  occasionally  witnessing  unexpected  discoveries 
that  produce  a  more  lively  sensation  from  their 
reaction  on  social  life,  is  led  to  suppose  that 
we  live  in  a  really  exceptional  epoch,  scored  by 
profound  crises  and  illustrated  by  extraordinary 
discoveries,  whose  singularity  surpasses  everything 
known  in  the  past.  Thus  we  often  hear  it  said 
that  physics,  in  particular,  has  of  late  years  under- 
gone a  veritable  revolution ;  that  all  its  principles 
have  been  made  new,  that  all  the  edifices  con- 
structed by  our  fathers  have  been  overthrown,  and 
that  on  the  field  thus  cleared  has  sprung  up  the 
most  abundant  harvest  that  has  ever  enriched  the 
domain  of  science. 

It  is   in   fact  true  that  the  crop  becomes  richer 
and    more    fruitful,    thanks    to    the    development 


T3E  XEW  :  PHYS  TOS  AND  ITS  EVOLUTION 


of  our  laboratories,  and  that  the  quantity  of 
seekers  has  considerably  increased  in  all  countries, 
while  their  quality  has  not  diminished.  We  should 
be  sustaining  an  absolute  paradox,  and  at  the  same 
time  committing  a  crying  injustice,  were  we  to 
contest  the  high  importance  of  recent  progress,  and 
to  seek  to  diminish  the  glory  of  contemporary 
physicists.  Yet  it  may  be  as  well  not  to  give  way 
to  exaggerations,  however  pardonable,  and  to  guard 
against  facile  illusions.  On  closer  examination  it 
will  be  seen  that  our  predecessors  might  at  several 
periods  in  history  have  conceived,  as  legitimately  as 
ourselves,  similar  sentiments  of  scientific  pride,  and 
have  felt  that  the  world  was  about  to  appear  to 
them  transformed  and  under  an  aspect  until  then 
absolutely  unknown. 

Let  us  take  an  example  which  is  salient  enough  ; 
for,  however  arbitrary  the  conventional  division  of 
time  may  appear  to  a  physicist's  eyes,  it  is  natural, 
when  instituting  a  comparison  between  two  epochs, 
to  choose  those  which  extend  over  a  space  of  half  a 
score  of  years,  and  are  separated  from  each  other  by 
the  gap  of  a  century.  Let  us,  then,  go  back  a 
hundred  years  and  examine  what  would  have  been 
the  state  of  mind  of  an  erudite  amateur  who  had  read 
and  understood  the  chief  publications  on  physical 
research  between  1800  and  1810. 

Let  us  suppose  that  this  intelligent  and  attentive 
spectator  witnessed  in  1800  the  discovery  of  the 


THE  EVOLUTION  OF   PHYSICS  3 

galvanic  battery  by  Volta.  He  might  from  that 
moment  have  felt  a  presentiment  that  a  prodigious 
transformation  was  about  to  occur  in  our  mode  of 
regarding  electrical  phenomena.  Brought  up  in  the 
ideas  of  Coulomb  and  Franklin,  he  might  till  then 
have  imagined  that  electricity  had  unveiled  nearly 
all  its  mysteries,  when  an  entirely  original  apparatus 
suddenly  gave  birth  to  applications  of  the  highest 
interest,  and  excited  the  blossoming  of  theories  of 
immense  philosophical  extent. 

In  the  treatises  on  physics  published  a  little  later, 
we  find  traces  of  the  astonishment  produced  by  this 
sudden  revelation  of  a  new  world.  "  Electricity," 
wrote  the  Abbe  Haiiy,  "  enriched  by  the  labour  of 
so  many  distinguished  physicists,  seemed  to  have 
reached  the  term  when  a  science  has  no  further  im- 
portant steps  before  it,  and  only  leaves  to  those  who 
cultivate  it  the  hope  of  confirming  the  discoveries 
of  their  predecessors,  and  of  casting  a  brighter 
light  on  the  truths  revealed.  One  would  have 
thought  that  all  researches  for  diversifying  the  results 
of  experiment  were  exhausted,  and  that  theory  itself 
could  only  be  augmented  by  the  addition  of  a  greater 
degree  of  precision  to  the  applications  of  principles 
already  known.  While  science  thus  appeared  to  be 
making  for  repose,  the  phenomena  of  the  convulsive 
movements  observed  by  Galvani  in  the  muscles  of 
a  frog  when  connected  by  metal  were  brought  to  the 
attention  and  astonishment  of  physicists.  .  .  .  Volta, 


4       THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

in  that  Italy  which  had  been  the  cradle  of  the 
new  knowledge,  discovered  the  principle  of  its  true 
theory  in  a  fact  which  reduces  the  explanation  of 
all  the  phenomena  in  question  to  the  simple  con- 
tact of  two  substances  of  different  nature.  This  fact 
became  in  his  hands  the  germ  of  the  admirable 
apparatus  to  which  its  manner  of  being  and  its 
fecundity  assign  one  of  the  chief  places  among 
those  with  which  the  genius  of  mankind  has  enriched 
physics." 

Shortly  afterwards,  our  amateur  would  learn 
that  Carlisle  and  Nicholson  had  decomposed  water 
by  the  aid  of  a  battery;  then,  that  Davy,  in  1803, 
had  produced,  by  the  help  of  the  same  battery,  a  quite 
unexpected  phenomenon,  and  had  succeeded  in  pre- 
paring metals  endowed  with  marvellous  properties, 
beginning  with  substances  of  an  earthy  appearance 
which  had  been  known  for  a  long  time,  but  whose 
real  nature  had  not  been  discovered. 

In  another  order  of  ideas,  surprises  as  prodigious 
would  wait  for  our  amateur.  Commencing  with 
1802,  he  might  have  read  the  admirable  series 
of  memoirs  which  Young  then  published,  and  might 
thereby  have  learned  how  the  study  of  the  phe- 
nomena of  diffraction  led  to  the  belief  that  the 
undulation  theory,  which,  since  the  works  of 
Newton  seemed  irretrievably  condemned,  was,  on 
the  contrary,  beginning  quite  a  new  life.  A  little 
later — in  1808 — he  might  have  witnessed  the  dis- 


THE  EVOLUTION  OF  PHYSICS  5 

covery  made  by  Mains  of  polarization  by  reflexion, 
and  would  have  been  able  to  note,  no  doubt  with 
stupefaction,  that  under  certain  conditions  a  ray  of 
light  loses  the  property  of  being  reflected. 

He  might  also  have  heard  of  one  Kurnford,  who 
was  then  promulgating  very  singular  ideas  on  the 
nature  of  heat,  who  thought  that  the  then  classical 
notions  might  be  false,  that  caloric  does  not  exist 
as  a  fluid,  and  who,  in  1804,  even  demonstrated  that 
heat  is  created  by  friction.  A  few  years  later 
he  would  learn  that  Charles  had  enunciated  a 
capital  law  on  the  dilatation  of  gases;  that  Pierre 
Prevost,  in  1809,  was  making  a  study,  full  of 
original  ideas,  on  radiant  heat.  In  the  meantime  he 
would  not  have  failed  to  read  volumes  iii.  and  iv. 
of  the  Mfoanique  cdeste  of  Laplace,  published  in  1804 
and  1805,  and  he  might,  no  doubt,  have  thought 
that  before  long  mathematics  would  enable  physical 
science  to  develop  with  unforeseen  safety. 

All  these  results  may  doubtless  be  compared  in 
importance  with  the  present  discoveries.  When 
strange  metals  like  potassium  and  sodium  were 
isolated  by  an  entirely  new  method,  the  astonish- 
ment must  have  been  on  a  par  with  that  caused 
in  our  time  by  the  magnificent  discovery  of  radium. 
The  polarization  of  light  is  a  phenomenon  as  un- 
doubtedly singular  as  the  existence  of  the  X  rays ; 
and  the  upheaval  produced  in  natural  philosophy  by 
the  theories  of  the  disintegration  of  matter  and  the 


6      THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

ideas  concerning  electrons  is  probably  not  more 
considerable  than  that  produced  in  the  theories 
of  light  and  heat  by  the  works  of  Young  and 
Eumford. 

If  we  now  disentangle  ourselves  from  contingencies, 
it  will  be  understood  that  in  reality  physical  science 
progresses  by  evolution  rather  than  by  revolution. 
Its  march  is  continuous.  The  facts  which  our  theories 
enable  us  to  discover,  subsist  and  are  linked  together 
long  after  these  theories  have  disappeared.  Out  of 
the  materials  of  former  edifices  overthrown,  new 
dwellings  are  constantly  being  reconstructed. 

The  labour  of  our  forerunners  never  wholly 
perishes.  The  ideas  of  yesterday  prepare  for  those 
of  to-morrow;  they  contain  them,  so  to  speak,  in 
potentia.  Science  is  in  some  sort  a  living  organism, 
which  gives  birth  to  an  indefinite  series  of  new 
beings  taking  the  places  of  the  old,  and  which 
evolves  according  to  the  nature  of  its  environment, 
adapting  itself  to  external  conditions,  and  healing 
at  every  step  the  wounds  which  contact  with 
reality  may  have  occasioned. 

Sometimes  this  evolution  is  rapid,  sometimes  it 
is  slow  enough  ;  but  it  obeys  the  ordinary  laws.  The 
wants  imposed  by  its  surroundings  create  certain 
organs  in  science.  The  problems  set  to  physicists  by 
the  engineer  who  wishes  to  facilitate  transport  or  to 
produce  better  illumination,  or  by  the  doctor  who 
seeks  to  know  how  such  and  such  a  remedy  acts, 


THE  EVOLUTION  OF  PHYSICS  7 

or,  again,  by  the  physiologist  desirous  of  understand- 
ing the  mechanism  of  the  gaseous  and  liquid  ex- 
changes between  the  cell  and  the  outer  medium, 
cause  new  chapters  in  physics  to  appear,  and  suggest 
researches  adapted  to  the  necessities  of  actual  life. 

The  evolution  of  the  different  parts  of  physics  does 
not,  however,  take  place  with  equal  speed,  because 
the  circumstances  in  which  they  are  placed  are  not 
equally  favourable.  Sometimes  a  whole  series  of 
questions  will  appear  forgotten,  and  will  live  only 
with  a  languishing  existence;  and  then  some  acci- 
dental circumstance  suddenly  brings  them  new  life, 
and  they  become  the  object  of  manifold  labours, 
engross  public  attention,  and  invade  nearly  the  whole 
domain  of  science. 

We  have  in  our  own  day  witnessed  such  a 
spectacle.  The  discovery  of  the  X  rays — a  discovery 
which  physicists  no  doubt  consider  as  the  logical 
outcome  of  researches  long  pursued  by  a  few  scholars 
working  in  silence  and  obscurity  on  an  otherwise 
much  neglected  subject — seemed  to  the  public  eye 
to  have  inaugurated  a  new  era  in  the  history  of 
physics.  If,  as  is  the  case,  however,  the  extra- 
ordinary scientific  movement  provoked  by  Kontgen's 
sensational  experiments  has  a  very  remote  origin,  it 
has,  at  least,  been  singularly  quickened  by  the  favour- 
able conditions  created  by  the  interest  aroused  in  its 
astonishing  applications  to  radiography. 

A  lucky  chance  has  thus  hastened  an  evolution 


8      THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

already  taking  place,  and  theories  previously  outlined 
have  received  a  singular  development.  Without 
wishing  to  yield  too  much  to  what  may  be  con- 
sidered a  whim  of  fashion,  we  cannot,  if  we  are  to 
note  in  this  book  the  stage  actually  reached  in  the 
continuous  march  of  physics,  refrain  from  giving  a 
clearly  preponderant  place  to  the  questions  suggested 
by  the  study  of  the  new  radiations.  At  the  present 
time  it  is  these  questions  which  move  us  the  most ; 
they  have  shown  us  unknown  horizons,  and  towards 
the  fields  recently  opened  to  scientific  activity  the 
daily  increasing  crowd  of  searchers  rushes  in  rather 
disorderly  fashion. 

One  of  the  most  interesting  consequences  of  the 
recent  discoveries  has  been  to  rehabilitate  in  the  eyes 
of  scholars,  speculations  relating  to  the  constitution 
of  matter,  and,  in  a  more  general  way,  metaphysical 
problems.  Philosophy  has,  of  course,  never  been 
completely  separated  from  science ;  but  in  times  past 
many  physicists  dissociated  themselves  from  studies 
which  they  looked  upon  as  unreal  word-squabbles, 
and  sometimes  not  unreasonably  abstained  from 
joining  in  discussions  which  seemed  to  them  idle 
and  of  rather  puerile  subtlety.  They  had  seen  the 
ruin  of  most  of  the  systems  built  up  a  priori  by 
daring  philosophers,  and  deemed  it  more  prudent  to 
listen  to  the  advice  given  by  Kirchhoff  and  "  to 
substitute  the  description  of  facts  for  a  sham 
explanation  of  nature." 


THE  EVOLUTION  OF  PHYSICS  9 

It  should  however  be  remarked  that  these  physicists 
somewhat  deceived  themselves  as  to  the  value  of 
their  caution,  and  that  the  mistrust  they  manifested 
towards  philosophical  speculations  did  not  preclude 
their  admitting,  unknown  to  themselves,  certain 
axioms  which  they  did  not  discuss,  but  which  are, 
properly  speaking,  metaphysical  conceptions.  They 
were  unconsciously  speaking  a  language  taught  them 
by  their  predecessors,  of  which  they  made  no  at- 
tempt to  discover  the  origin.  It  is  thus  that  it  was 
readily  considered  evident  that  physics  must  neces- 
sarily some  day  re-enter  the  domain  of  mechanics, 
and  thence  it  was  postulated  that  everything  in 
nature  is  due  to  movement.  We,  further,  accepted 
the  principles  of  the  classical  mechanics  without 
discussing  their  legitimacy. 

This  state  of  mind  was,  even  of  late  years,  that 
of  the  most  illustrious  physicists.  It  is  mani- 
fested, quite  sincerely  and  without  the  slightest 
reserve,  in  all  the  classical  works  devoted  to  physics. 
Thus  Verdet,  an  illustrious  professor  who  has  had  the 
greatest  and  most  happy  influence  on  the  intellectual 
formation  of  a  whole  generation  of  scholars,  and  whose 
works  are  even  at  the  present  day  very  often  con- 
sulted, wrote  :  "  The  true  problem  of  the  physicist  is 
always  to  reduce  all  phenomena  to  that  which  seems 
to  us  the  simplest  and  clearest,  that  is  to  say,  to 
movement."  In  his  celebrated  course  of  lectures  at 
1'Scole  Poly  technique,  Jamin  likewise  said  :  "Physics 


io    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

will  one  day  form  a  chapter  of  general  mechanics ; " 
and  in  the  preface  to  his  excellent  course  of  lectures 
on  physics,  M.  Violle,  in  1884,  thus  expresses  him- 
self :  "  The  science  of  nature  tends  towards  mechanics 
by  a  necessary  evolution,  the  physicist  being  able  to 
establish  solid  theories  only  on  the  laws  of  move- 
ment. "  The  same  idea  is  again  met  with  in  the  words 
of  Cornu  in  1896  :  "  The  general  tendency  should  be 
to  show  how  the  facts  observed  and  the  pheno- 
mena measured,  though  first  brought  together  by 
empirical  laws,  end,  by  the  impulse  of  successive 
progressions,  in  coming  under  the  general  laws  of 
rational  mechanics ; "  and  the  same  physicist  showed 
clearly  that  in  his  mind  this  connexion  of  phenomena 
with  mechanics  had  a  deep  and  philosophical  reason, 
when,  in  the  fine  discourse  pronounced  by  him  at  the 
opening  ceremony  of  the  Congres  de  Physique  in 
1900,  he  exclaimed:  "  The  mind  of  Descartes  soars 
over  modern  physics,  or  rather,  I  should  say,  he  is 
their  luminary.  The  further  we  penetrate  into  the 
knowledge  of  natural  phenomena,  the  clearer  and  the 
more  developed  becomes  the  bold  Cartesian  conception 
regarding  the  mechanism  of  the  universe.  There  is 
nothing  in  the  physical  world  but  matter  and 
movement." 

*  If  we  adopt  this  conception,  we  are  led  to  con- 
struct mechanical  representations  of  the  material 
world,  and  to  imagine  movements  in  the  different 
parts  of  bodies  capable  of  reproducing  all  the  mani- 


THE  EVOLUTION   OF  PHYSICS  11 

festations  of  nature.  The  kinematic  knowledge  of 
these  movements,  that  is  to  say,  the  determination 
of  the  position,  speed,  and  acceleration  at  a  given 
moment  of  all  the  parts  of  the  system,  or,  on  the 
other  hand,  their  dynamical  study,  enabling  us  to 
know  what  is  the  action  of  these  parts  on  each 
other,  would  then  be  sufficient  to  enable  us  to 
foretell  all  that  can  occur  in  the  domain  of  nature. 

This  was  the  great  thought  clearly  expressed  by 
the  Encyclopedists  of  the  eighteenth  century;  and 
if  the  necessity  of  interpreting  the  phenomena 
of  electricity  or  light  led  the  physicists  of  last 
century  to  imagine  particular  fluids  which  seemed 
to  obey  with  some  difficulty  the  ordinary  rules  of 
mechanics,  these  physicists  still  continued  to  retain 
their  hope  in  the  future,  and  to  treat  the  idea  of 
Descartes  as  an  ideal  to  be  reached  sooner  or  later. 

Certain  scholars — particularly  those  of  the  English 
School — outrunning  experiment,  and  pushing  things 
to  extremes,  took  pleasure  in  proposing  very 
curious  mechanical  models  which  were  often 
strange  images  of  reality.  The  most  illustrious  of 
them,  Lord  Kelvin,  may  be  considered  as  their 
representative  type,  and  he  has  himself  said :  "  It 
seems  to  me  that  the  true  sense  of  the  question,  Do 
we  or  do  we  not  understand  a  particular  subject  in 
physics  ?  is — Can  we  make  a  mechanical  model  which 
corresponds  to  it  ?  I  am  never  satisfied  so  long  as  I 
have  been  unable  to  make  a  mechanical  model  of  the 


12     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

object.  If  I  am  able  to  do  so,  I  understand  it.  If 
I  cannot  make  such  a  model,  I  do  not  understand 
it."  But  it  must  be  acknowledged  that  some  of 
the  models  thus  devised  have  become  excessively 
complicated,  and  this  complication  has  for  a  long 
time  discouraged  all  but  very  bold  minds.  In 
addition,  when  it  became  a  question  of  penetrating 
into  the  mechanism  of  molecules,  and  we  were  no 
longer  satisfied  to  look  at  matter  as  a  mass,  the 
mechanical  solutions  seemed  undetermined  and  the 
stability  of  the  edifices  thus  constructed  was  in- 
sufficiently demonstrated. 

Keturning  then  to  our  starting-point,  many 
contemporary  physicists  wish  to  subject  Descartes' 
idea  to  strict  criticism.  From  the  philoso- 
phical point  of  view,  they  first  enquire  whether 
it  is  really  demonstrated  that  there  exists  nothing 
else  in  the  knowable  than  matter  and  move- 
ment. They  ask  themselves  whether  it  is  not 
habit  and  tradition  in  particular  which  lead  us  to 
ascribe  to  mechanics  the  origin  of  phenomena. 
Perhaps  also  a  question  of  sense  here  comes  in. 
Our  senses,  which  are,  after  all,  the  only  windows 
open  towards  external  reality,  give  us  a  view  of  one 
side  of  the  world  only ;  evidently  we  only  know  the 
universe  by  the  relations  which  exist  between  it  and 
our  organisms,  and  these  organisms  are  peculiarly 
sensitive  to  movement. 

Nothing,  however,  proves  that  those  acquisitions 


THE  EVOLUTION  OF   PHYSICS  13 

which  are  the  most  ancient  in  historical  order  ought, 
in  the  development  of  science,  to  remain  the  basis 
of  our  knowledge.  Nor  does  any  theory  prove  that 
our  perceptions  are  an  exact  indication  of  reality. 
Many  reasons,  on  the  contrary,  might  be  invoked 
which  tend  to  compel  us  to  see  in  nature  phenomena 
which  cannot  be  reduced  to  movement. 

Mechanics  as  ordinarily  understood  is  the  study 
of  reversible  phenomena.  If  there  be  given  to  the 
parameter  which  represents  time,1  and  which  has 
assumed  increasing  values  during  the  duration  of 
the  phenomena,  decreasing  values  which  make  it  go 
the  opposite  way,  the  whole  system  will  again  pass 
through  exactly  the  same  stages  as  before,  and  all 
the  phenomena  will  unfold  themselves  in  reversed 
order.  In  physics,  the  contrary  rule  appears  very 
general,  and  reversibility  generally  does  not  exist. 
It  is  an  ideal  and  limited  case,  which  may  be  some- 
times approached,  but  can  never,  strictly  speaking, 
be  met  with  in  its  entirety.  No  physical  pheno- 
menon ever  recommences  in  an  identical  manner 
if  its  direction  be  altered.  It  is  true  that  certain 
mathematicians  warn  us  that  a  mechanics  can  be 
devised  in  which  reversibility  would  no  longer  be 
the  rule,  but  the  bold  attempts  made  in  this  direc- 
tion are  not  wholly  satisfactory. 

On  the  other  hand,  it  is  established  that  if  a 
mechanical  explanation  of  a  phenomenon  can  be 
1  I.e.  the  time-curve. — ED. 


i4     THE  NEW   PHYSICS  AND  ITS  EVOLUTION 

given,  we  can  find  an  infinity  of  others  which 
likewise  account  for  all  the  peculiarities  revealed 
by  experiment.  But,  as  a  matter  of  fact,  no  one 
has  ever  succeeded  in  giving  an  indisputable 
mechanical  representation  of  the  whole  physical 
world.  Even  were  we  disposed  to  admit  the 
strangest  solutions  of  the  problem;  to  consent,  for 
example,  to  be  satisfied  with  the  hidden  systems 
devised  by  Helmholtz,  whereby  we  ought  to  divide 
variable  things  into  two  classes,  some  accessible,  and 
the  others  now  and  for  ever  unknown,  we  should 
never  manage  to  construct  an  edifice  to  contain  all- 
the  known  facts.  Even  the  very  comprehensive 
mechanics  of  a  Hertz  fails  where  the  classical 
mechanics  has  not  succeeded. 

Deeming  this  check  irremediable,  many  contem- 
porary physicists  give  up  attempts  which  they  look 
upon  as  condemned  beforehand,  and  adopt,  to  guide 
them  in  their  researches,  a  method  which  at  first 
sight  appears  much  more  modest,  and  also  much 
more  sure.  They  make  up  their  minds  not  to  see  at 
once  to  the  bottom  of  things ;  they  no  longer  seek  to 
suddenly  strip  the  last  veils  from  nature,  and  to 
divine  her  supreme  secrets ;  but  they  work  prudently 
and  advance  but  slowly,  while  on  the  ground  thus 
conquered  foot  by  foot  they  endeavour  to  establish 
themselves  firmly.  They  study  the  various  magni- 
tudes directly  accessible  to  their  observation  without 
busying  themselves  as  to  their  essence.  They 


THE  EVOLUTION   OF  PHYSICS  15 

measure  quantities  of  heat  and  of  temperature,  differ- 
ences of  potential,  currents,  and  magnetic  fields ; 
and  then,  varying  the  conditions,  apply  the  rules  of 
experimental  method,  and  discover  between  these 
magnitudes  mutual  relations,  while  they  thus  suc- 
ceed in  enunciating  laws  which  translate  and  sum 
up  their  labours. 

These  empirical  laws,  however,  themselves  bring 
about  by  induction  the  promulgation  of  more  general 
laws,  which  are  termed  principles.  These  principles 
are  originally  only  the  results  of  experiments,  and  ex- 
periment allows  them  besides  to  be  checked,  and  their 
more  or  less  high  degree  of  generality  to  be  verified. 
When  they  have  been  thus  definitely  established, 
they  may  serve  as  fresh  starting-points,  and,  by 
deduction,  lead  to  very  varied  discoveries. 

The  principles  which  govern  physical  science 
are  few  in  number,  and  their  very  general  form  gives 
them  a  philosophical  appearance,  while  we  cannot 
long  resist  the  temptation  of  regarding  them  as 
metaphysical  dogmas.  It  thus  happens  that  the 
least  bold  physicists,  those  who  have  wanted  to  show 
themselves  the  most  reserved,  are  themselves  led  to 
forget  the  experimental  character  of  the  laws  they 
have  propounded,  and  to  see  in  them  imperious 
beings  whose  authority,  placed  above  all  verification, 
can  no  longer  be  discussed. 

Others,  on  the  contrary,  carry  prudence  to  the 
extent  of  timidity.  They  desire  to  grievously 


16     THE  NEW   PHYSICS  AND   ITS   EVOLUTION 

limit  the  field  of  scientific  investigation,  and  they 
assign  to  science  a  too  restricted  domain.  They 
content  themselves  with  representing  phenomena 
by  equations,  and  think  that  they  ought  to  submit 
to  calculation  magnitudes  experimentally  determined, 
without  asking  themselves  whether  these  calculations 
retain  a  physical  meaning.  They  are  thus  led  to 
reconstruct  a  physics  in  which  there  again  appears 
the  idea  of  quality,  understood,  of  course,  not  in 
the  scholastic  sense,  since  from  this  quality  we  can 
argue  with  some  precision  by  representing  it  under 
numerical  symbols,  but  still  constituting  an  element 
of  differentiation  and  of  heterogeneity. 

Notwithstanding  the  errors  they  may  lead  to  if 
carried  to  excess,  both  these  doctrines  render,  as 
a  whole,  most  important  service.  It  is  no  bad 
thing  that  these  contradictory  tendencies  should 
subsist,  for  this  variety  in  the  conception  of  pheno- 
mena gives  to  actual  science  a  character  of  intense 
life  and  of  veritable  youth,  capable  of  impassioned 
efforts  towards  the  truth.  Spectators  who  see  such 
moving  and  varied  pictures  passing  before  them, 
experience  the  feeling  that  there  no  longer  exist 
systems  fixed  in  an  immobility  which  seems  that 
of  death.  They  feel  that  nothing  is  unchangeable ; 
that  ceaseless  transformations  are  taking  place  before 
their  eyes ;  and  that  this  continuous  evolution  and 
perpetual  change  are  the  necessary  conditions  of 
progress. 


THE  EVOLUTION  OF  PHYSICS  17 

A  great  number  of  seekers,  moreover,  show 
themselves  on  their  own  account  perfectly  eclectic. 
They  adopt,  according  to  their  needs,  such  or  such  a 
manner  of  looking  at  nature,  and  do  not  hesitate  to 
utilize  very  different  images  when  they  appear  to 
them  useful  and  convenient.  And,  without  doubt, 
they  are  not  wrong,  since  these  images  are  only 
symbols  convenient  for  language.  They  allow  facts 
to  be  grouped  and  associated,  but  only  present  a 
fairly  distant  resemblance  with  the  objective  reality. 
Hence  it  is  not  forbidden  to  multiply  and  to  modify 
them  according  to  circumstances.  The  really  essen- 
tial thing  is  to  have,  as  a  guide  through  the  unknown, 
a  map  which  certainly  does  not  claim  to  represent 
all  the  aspects  of  nature,  but  which,  having  been 
drawn  up  according  to  predetermined  rules,  allows  us 
to  follow  an  ascertained  road  in  the  eternal  journey 
towards  the  truth. 

Among  the  provisional  theories  which  are  thus 
willingly  constructed  by  scholars  on  their  journey, 
like  edifices  hastily  run  up  to  receive  an  unfore- 
seen harvest,  some  still  appear  very  bold  and  very 
singular.  Abandoning  the  search  after  mechanical 
models  for  all  electrical  phenomena,  certain  physicists 
reverse,  so  to  speak,  the  conditions  of  the  problem, 
and  ask  themselves  whether,  instead  of  giving  a 
mechanical  interpretation  to  electricity,  they  may 
not,  on  the  contrary,  give  an  electrical  interpretation 
to  the  phenomena  of  matter  and  motion,  and  thus 

2 


1 8     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

merge  mechanics  itself  in  electricity.  One  thus  sees 
dawning  afresh  the  eternal  hope  of  co-ordinating  all 
natural  phenomena  in  one  grandiose  and  imposing 
synthesis.  Whatever  may  be  the  fate  reserved  for 
such  attempts,  they  deserve  attention  in  the  highest 
degree ;  and  it  is  desirable  to  examine  them  carefully 
if  we  wish  to  have  an  exact  idea  of  the  tendencies  of 
modern  physics. 


CHAPTER  II 
MEASUREMENTS 

§  1.  METROLOGY 

NOT  so  very  long  ago,  the  scholar  was  often  content 
with  qualitative  observations.  Many  phenomena 
were  studied  without  much  trouble  being  taken  to 
obtain  actual  measurements.  But  it  is  now  becom- 
ing more  and  more  understood  that  to  establish  the 
relations  which  exist  between  physical  magnitudes, 
and  to  represent  the  variations  of  these  magnitudes 
by  functions  which  allow  us  to  use  the  power  of 
mathematical  analysis,  it  is  most  necessary  to  express 
each  magnitude  by  a  definite  number. 

Under  these  conditions  alone  can  a  magnitude  be 
considered  as  effectively  known.  "  I  often  say,"  Lord 
Kelvin  has  said,  "  that  if  you  can  measure  that  of 
which  you  are  speaking  and  express  it  by  a  number 
you  know  something  of  your  subject;  but  if  you 
cannot  measure  it  nor  express  it  by  a  number,  your 
knowledge  is  of  a  sorry  kind  and  hardly  satisfactory. 
It  may  be  the  beginning  of  the  acquaintance,  but  you 


20    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

are  hardly,  in  your  thoughts,  advanced  towards 
science,  whatever  the  subject  may  be." 

It  has  now  become  possible  to  measure  exactly 
the  elements  which  enter  into  nearly  all  physical 
phenomena,  and  these  measurements  are  taken  with 
ever  increasing  precision.  Every  time  a  chapter  in 
science  progresses,  science  shows  itself  more  exacting ; 
it  perfects  its  means  of  investigation,  it  demands 
more  and  more  exactitude,  and  one  of  the  most 
striking  features  of  modern  physics  is  this  constant 
care  for  strictness  and  clearness  in  experimentation. 

A  veritable  science  of  measurement  has  thus  been 
constituted  which  extends  over  all  parts  of  the  domain 
of  physics.  This  science  has  its  rules  and  its  methods ; 
it  points  out  the  best  processes  of  calculation,  and 
teaches  the  method  of  correctly  estimating  errors 
and  taking  account  of  them.  It  has  perfected  the 
processes  of  experiment,  co-ordinated  a  large  number 
of  results,  and  made  possible  the  unification  of 
standards.  It  is  thanks  to  it  that  the  system  of 
measurements  unanimously  adopted  by  physicists 
has  been  formed. 

At  the  present  day  we  designate  more  peculiarly 
by  the  name  of  metrology  that  part  of  the  science 
of  measurements  which  devotes  itself  specially  to 
the  determining  of  the  prototypes  representing 
the  fundamental  units  of  dimension  and  mass,  and 
of  the  standards  of  the  first  order  which  are 
derived  from  them.  If  all  measurable  quantities, 


MEASUREMENTS  21 

as  was  long  thought  possible,  could  be  reduced  to 
the  magnitudes  of  mechanics,  metrology  would  thus 
be  occupied  with  the  essential  elements  entering 
into  all  phenomena,  and  might  legitimately  claim  the 
highest  rank  in  science.  But  even  when  we  suppose 
that  some  magnitudes  can  never  be  connected  with 
mass,  length,  and  time,  it  still  holds  a  preponderat- 
ing place,  and  its  progress  finds  an  echo  throughout 
the  whole  domain  of  the  natural  sciences.  It  is 
therefore  well,  in  order  to  give  an  account  of  the 
general  progress  of  physics,  to  examine  at  the  outset 
the  improvements  which  have  been  effected  in  these 
fundamental  measurements,  and  to  see  what  precision 
these  improvements  have  allowed  us  to  attain. 

§  2.  THE  MEASURE  OF  LENGTH 

To  measure  a  length  is  to  compare  it  with  another 
length  taken  as  unity.  Measurement  is  therefore 
a  relative  operation,  and  can  only  enable  us  to  know 
ratios.  Did  both  the  length  to  be  measured  and 
the  unit  chosen  happen  to  vary  simultaneously  and 
in  the  same  degree,  we  should  perceive  no  change. 
Moreover,  the  unit  being,  by  definition,  the  term 
of  comparison,  and  not  being  itself  comparable 
with  anything,  we  have  theoretically  no  means  of 
ascertaining  whether  its  length  varies. 

If,  however,  we  were  to  note  that,  suddenly  and 
in  the  same  proportions,  the  distance  between  two 
points  on  this  earth  had  increased,  that  all  the 


22     THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

planets  had  moved  further  from  each  other,  that  all 
objects  around  us  had  become  larger,  that  we  ourselves 
had  become  taller,  and  that  the  distance  travelled 
by  light  in  the  duration  of  a  vibration  had  become 
greater,  we  should  not  hesitate  to  think  ourselves  the 
victims  of  an  illusion,  that  in  reality  all  these  dis- 
tances had  remained  fixed,  and  that  all  these  appear- 
ances were  due  to  a  shortening  of  the  rule  which  we 
had  used  as  the  standard  for  measuring  the  lengths. 
From  the  mathematical  point  of  view,  it  may  be 
considered  that  the  two  hypotheses  are  equivalent ; 
all  has  lengthened  around  us,  or  else  our  standard 
has   become   less.     But  it  is  no  simple  question  of 
convenience  and  simplicity  which  leads  us  to  reject 
the  one  supposition  and  to  accept  the  other ;  it  is 
right  in  this  case  to  listen  to  the  voice  of  common 
sense,  and  those  physicists  who  have  an  instinctive 
trust  in  the  notion  of  an  absolute  length  are  perhaps 
not  wrong.     It  is  only  by  choosing   our  unit  from 
those    which    at    all    times    have    seemed    to    all 
men  the  most  invariable,  that  we  are  able  in  our 
experiments   to   note   that    the   same  causes  acting 
under  identical  conditions  always  produce  the  same 
effects.     The  idea  of  absolute  length  is  derived  from 
the  principle  of  causality  ;  and  our  choice  is  forced 
upon  us  by  the  necessity  of  obeying  this  principle, 
which  we  cannot  reject  without  declaring   by  that 
very  act  all  science  to  be  impossible. 

Similar   remarks   might   be  made  with  regard  to 


MEASUREMENTS  23 

the  notions  of  absolute  time  and  absolute  movement. 
They  have  been  put  in  evidence  and  set  forth  very 
forcibly  by  a  learned  and  profound  mathematician, 
M.  Painleve. 

On  the  particularly  clear  example  of  the  measure 
of  length,  it  is  interesting  to  follow  the  evolution 
of  the  methods  employed,  and  to  run  through  the 
history  of  the  progress  in  precision  from  the  time 
that  we  have  possessed  authentic  documents  relating 
to  this  question.  This  history  has  been  written  in 
a  masterly  way  by  one  of  the  physicists  who  have 
in  our  days  done  the  most  by  their  personal  labours 
to  add  to  it  glorious  pages.  M.  Benoit,  the  learned 
Director  of  the  International  Bureau  of  Weights  and 
Measures,  has  furnished  in  various  reports  very 
complete  details  on  the  subject,  from  which  I  here 
borrow  the  most  interesting. 

We  know  that  in  France  the  fundamental 
standard  for  measures  of  length  was  for  a  long 
time  the  Toise  du  Chdtelet,  a  kind  of  callipers 
formed  of  a  bar  of  iron  which  in  1668  was  embedded 
in  the  outside  wall  of  the  Chatelet,  at  the  foot  of 
the  staircase.  This  bar  had  at  its  extremities  two 
projections  with  square  faces,  and  all  the  toises  of 
commerce  had  to  fit  exactly  between  them.  Such 
a  standard,  roughly  constructed,  and  exposed  to  all 
the  injuries  of  weather  and  time,  offered  very  slight 
guarantees  either  as  to  the  permanence  or  the 
correctness  of  its  copies.  Nothing,  perhaps,  can 


24     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

better  convey  an  idea  of  the  importance  of  the 
modifications  made  in  the  methods  of  experimental 
physics  than  the  easy  comparison  between  so 
rudimentary  a  process  and  the  actual  measurements 
effected  at  the  present  time. 

The  Toise  du  Chdtelet,  notwithstanding  its  evident 
faults,  was   employed   for  nearly  a  hundred  years  ; 
in  1766  it  was  replaced  by  the  Toise  du  Pfrou,  so 
called  because  it  had  served  for  the  measurements  of 
the  terrestrial  arc  effected   in   Peru   from    1735  to 
1739   by   Bouguer,  La  Condamine,  and  Godin.     At 
that  time,  according  to  the  comparisons  made  between 
this  new  toise  and  the  Toise  du  Nord,  which  had  also 
been  used  for   the   measurement  of  an  arc  of   the 
meridian,  an  error  of  the  tenth  part  of  a  millimetre 
in  measuring  lengths    of  the  order  of  a  metre  was 
considered   quite   unimportant.     At  the  end  of  the 
eighteenth   century,  Delambre,  in  his  work   Sur  la 
Base  du  Systeme  metrique  decimal,  clearly  gives   us 
to  understand  that  magnitudes  of  the  order  of  the 
hundredth  of  a  millimetre  appear  to  him  incapable 
of  observation,  even  in  scientific  researches  of  the 
highest  precision.     At  the  present  date  the  Interna- 
tional Bureau  of  Weights  and  Measures  guarantees, 
in  the  determination  of  a  standard  of  length  com- 
pared with  the  metre,  an  approximation  of  two  or 
three  ten-thousandths  of  a  millimetre,  and  even  a 
little  more  under  certain  circumstances. 

This   very   remarkable    progress    is    due   to   the 


MEASUREMENTS  25 

improvements  in  the  method  of  comparison  on  the 
one  hand,  and  in  the  manufacture  of  the  standard 
on  the  other.  M.  Benoit  rightly  points  out  that 
a  kind  of  competition  has  been  set  up  between  the 
standard  destined  to  represent  the  unit  with  its 
subdivisions  and  multiples  and  the  instrument 
charged  with  observing  it,  comparable,  up  to  a  certain 
point,  with  that  which  in  another  order  of  ideas 
goes  on  between  the  gun  and  the  armour-plate. 

The  measuring  instrument  of  to-day  is  an  instru- 
ment of  comparison  constructed  with  meticulous  care, 
which  enables  us  to  do  away  with  causes  of  error 
formerly  ignored,  to  eliminate  the  action  of  external 
phenomena,  and  to  withdraw  the  experiment  from 
the  influence  of  even  the  personality  of  the  observer. 
This  standard  is  no  longer,  as  formerly,  a  flat  rule, 
weak  and  fragile,  but  a  rigid  bar,  incapable  of  de- 
formation, in  which  the  material  is  utilised  in  the 
best  conditions  of  resistance.  For  a  standard  with 
ends  has  been  substituted  a  standard  with  marks, 
which  permits  much  more  precise  definition  and 
can  be  employed  in  optical  processes  of  observation 
alone ;  that  is,  in  processes  which  can  produce  in  it 
no  deformation  and  no  alteration.  Moreover,  the 
marks  are  traced  on  the  plane  of  the  neutral  fibres  1 
exposed,  and  the  invariability  of  their  distance  apart 

1  The  author  seems  to  refer  to  the  fact  that  in  the  standard 
metre,  the  measurement  is  taken  from  the  central  one  of  three 
marks  at  each  end  of  the  bar.  The  transverse  section  of  the 
bar  is  an  X,  and  the  reading  is  made  by  a  microscope. — ED. 


26    THE  NEW   PHYSICS  AND  ITS  EVOLUTION 

is  thus  assured,  even  when  a  change  is  made  in  the 
way  the  rule  is  supported. 

Thanks  to  studies  thus  systematically  pursued,  we 
have  succeeded  in  the  course  of  a  hundred  years 
in  increasing  the  precision  of  measures  in  the  pro- 
portion of  a  thousand  to  one,  and  we  may  ask 
ourselves  whether  such  an  increase  will  continue 
in  the  future.  No  doubt  progress  will  not  be 
stayed  ;  but  if  we  keep  to  the  definition  of  length 
by  a  material  standard,  it  would  seem  that  its  pre- 
cision cannot  be  considerably  increased.  We  have 
nearly  reached  the  limit  imposed  by  the  necessity 
of  making  strokes  of  such  a  thickness  as  to  be 
observable  under  the  microscope. 

It  may  happen,  however,  that  we  shall  be  brought 
one  of  these  days  to  a  new  conception  of  the  measure 
of  length,  and  that  very  different  processes  of 
determination  will  be  thought  of.  If  we  took  as 
unit,  for  instance,  the  distance  covered  by  a  given 
radiation  during  a  vibration,  the  optical  processes 
would  at  once  admit  of  much  greater  precision. 

Thus  Fizeau,  the  first  to  have  this  idea,  says : 
"  A  ray  of  light,  with  its  series  of  undulations  of 
extreme  tenuity  but  perfect  regularity,  may  be  con- 
sidered as  a  micrometer  of  the  greatest  perfection, 
and  particularly  suitable  for  determining  length." 
But  in  the  present  state  of  things,  since  the  legal 
and  customary  definition  of  the  unit  remains  a 
material  standard,  it  is  not  enough  to  measure 


MEASUREMENTS  27 

length  in  terms  of  wave-lengths,  and  we  must  also 
know  the  value  of  the^e  wave-lengths  in  terms  of 
the  standard  prototype  of  the  metre. 

This  was  determined  in  1894  by  M.  Michelson 
and  M.  Benoit  in  an  experiment  which  will  remain 
classic.  The  two  physicists  measured  a  standard 
length  of  about  ten  centimetres,  first  in  terms  of  the 
wave-lengths  of  the  red,  green,  and  blue  radiations 
of  cadmium,  and  then  in  terms  of  the  standard 
metre.  The  great  difficulty  of  the  experiment  pro- 
ceeds from  the  vast  difference  which  exists  between 
the  lengths  to  be  compared,  the  wave-lengths  barely 
amounting  to  half  a  micron  ; 1  the  process  employed 
consisted  in  noting,  instead  of  this  length,  a  length 
easily  made  about  a  thousand  times  greater,  namely, 
the  distance  between  the  fringes  of  interference. 

In  all  measurement,  that  is  to  say  in  every 
determination  of  the  relation  of  a  magnitude  to  the 
unit,  there  has  to  be  determined  on  the  one  hand 
the  whole,  and  on  the  other  the  fractional  part  of 
this  ratio,  and  naturally  the  most  delicate  determina- 
tion is  generally  that  of  this  fractional  part.  In 
optical  processes  the  difficulty  is  reversed.  The 
fractional  part  is  easily  known,  while  it  is  the  high 
figure  of  the  number  representing  the  whole  which 
becomes  a  very  serious  obstacle.  It  is  this  obstacle 
which  MM.  Michelson  and  Benoit  overcame  with 
admirable  ingenuity. 

1  I-e>  2  0*00  °f  a  millimetre. — ED. 


28     THE   NEW   PHYSICS  AND   ITS  EVOLUTION 

By  making  use  of  a  somewhat  similar  idea,  M.  Mace 
de  Lepinay  and  MM.  Perot  and  Fabry,  have  lately 
effected  by  optical  methods,  measurements  of  the 
greatest  precision,  and  no  doubt  further  progress 
may  still  be  made.  A  day  may  perhaps  come  when 
a  material  standard  will  be  given  up,  and  it  may 
perhaps  even  be  recognised  that  such  a  standard  in 
time  changes  its  length  by  molecular  strain,  and  by 
wear  and  tear :  and  it  will  be  further  noted  that,  in 
accordance  with  certain  theories  which  will  be 
noticed  later  on,  it  is  not  invariable  when  its 
orientation  is  changed. 

For  the  moment,  however,  the  need  of  any  change 
in  the  definition  of  the  unit  is  in  no  way  felt ;  we 
must,  on  the  contrary,  hope  that  the  use  of  the 
unit  adopted  by  the  physicists  of  the  whole  world 
will  spread  more  and  more.  It  is  right  to  remark 
that  a  few  errors  still  occur  with  regard  to  this 
unit,  and  that  these  errors  have  been  facilitated  by 
incoherent  legislation.  France  herself,  though  she 
was  the  admirable  initiator  of  the  metrical  system, 
has  for  too  long  allowed  a  very  regrettable  confusion 
to  exist ;  and  it  cannot  be  noted  without  a  certain 
sadness  that  it  was  not  until  the  llth  July  1903 
that  a  law  was  promulgated  re-establishing  the 
agreement  between  the  legal  and  the  scientific 
definition  of  the  metre. 

Perhaps  it  may  not  be  useless  to  briefly  indi- 
cate here  the  reasons  of  the  disagreement  which 


MEASUKEMENTS  29 

had  taken  place.  Two  definitions  of  the  metre  can 
be,  and  in  fact  were  given.  One  had  for  its  basis 
the  dimensions  of  the  earth,  the  other  the  length  of 
the  material  standard.  In  the  minds  of  the  founders 
of  the  metrical  system,  the  first  of  these  was  the  true 
definition  of  the  unit  of  length,  the  second  merely 
a  simple  representation.  It  was  admitted,  however, 
that  this  representation  had  been  constructed  in  a 
manner  perfect  enough  for  it  to  be  nearly  impossible 
to  perceive  any  difference  between  the  unit  and  its 
representation,  and  for  the  practical  identity  of  the 
two  definitions  to  be  thus  assured.  The  creators 
of  the  metrical  system  were  persuaded  that  the 
measurements  of  the  meridian  effected  in  their 
day  could  never  be  surpassed  in  precision ;  and 
on  the  other  hand,  by  borrowing  from  nature  a 
definite  basis,  they  thought  to  take  from  the  defini- 
tion of  the  unit  some  of  its  arbitrary  character, 
and  to  ensure  the  means  of  again  finding  the  same 
unit  if  by  any  accident  the  standard  became  altered. 
Their  confidence  in  the  value  of  the  processes 
they  had  seen  employed  was  exaggerated,  and  their 
mistrust  of  the  future  unjustified.  This  example 
shows  how  imprudent  it  is  to*  endeavour  to  fix 
limits  to  progress.  It  is  an  error  to  think  the 
march  of  science  can  be  stayed  ;  and  in  reality  it 
is  now  known  that  the  ten-millionth  part  of  the 
quarter  of  the  terrestrial  meridian  is  longer  than 
the  metre  by  0187  millimetres.  But  contemporary 


30     THE  NEW  PHYSICS  AND  ITS   EVOLUTION 

physicists  do  not  fall  into  the  same  error  as  their 
forerunners,  and  they  regard  the  present  result 
as  merely  provisional.  They  guess,  in  fact,  that 
new  improvements  will  be  effected  in  the  art  of 
measurement;  thev  know  that  geodesical  processes, 
though  much  improved  in  our  days,  have  still  much 
to  do  to  attain  the  precision  displayed  in  the  con- 
struction and  determination  of  standards  of  the  first 
order ;  and  consequently  they  do  not  propose  to  keep 
the  ancient  definition,  which  would  lead  to  having 
for  unit  a  magnitude  possessing  the  grave  defect 
from  a  practical  point  of  view  of  being  constantly 
variable. 

We  may  even  consider  that,  looked  at  theo- 
retically, its  permanence  would  not  be  assured. 
Nothing,  in  fact,  proves  that  sensible  variations  may 
not  in  time  be  produced  in  the  value  of  an  arc  of  the 
meridian,  and  serious  difficulties  may  arise  regarding 
the  probable  inequality  of  the  various  meridians. 

For  all  these  reasons,  the  idea  of  finding  a  natural 
unit  has  been  gradually  abandoned,  and  we  have 
become  resigned  to  accepting  as  a  fundamental  unit 
an  arbitrary  and  conventional  length  having  a 
material  representation  recognised  by  universal  con- 
sent ;  and  it  was  this  unit  which  was  consecrated  by 
the  following  law  of  the  llth  July  1903  :— 

"  The  standard  prototype  of  the  metrical  system  is 
the  international  metre,  which  has  been  sanctioned  by 
the  General  Conference  on  Weights  and  Measures." 


MEASUREMENTS  31 

§  3.  THE  MEASURE  OF  MASS 

On  the  subject  of  measures  of  mass,  similar  re- 
marks to  those  on  measures  of  length  might  be 
made.  The  confusion  here  was  perhaps  still  greater, 
because,  to  the  uncertainty  relating  to  the  fixing  of 
the  unit,  was  added  some  indecision  on  the  very 
nature  of  the  magnitude  defined.  In  law,  as  in 
ordinary  practice,  the  notions  of  weight  and  of  mass 
were  not,  in  fact,  separated  with  sufficient  clearness. 

They  represent,  however,  two  essentially  different 
things.  Mass  is  the  characteristic  of  a  quantity  of 
matter ;  it  depends  neither  on  the  geographical  posi- 
tion one  occupies  nor  on  the  altitude  to  which  one 
may  rise ;  it  remains  invariable  so  long  as  nothing 
material  is  added  or  taken  away.  Weight  is  the 
action  which  gravity  has  upon  the  body  under  con- 
sideration ;  this  action  does  not  depend  solely  on 
the  body,  but  on  the  earth  as  well ;  and  when  it  is 
changed  from  one  spot  to  another,  the  weight  changes, 
because  gravity  varies  with  latitude  and  altitude. 

These  elementary  notions,  to-day  understood  even 
by  young  beginners,  appear  to  have  been  for.  a  long 
time  indistinctly  grasped.  The  distinction  remained 
confused  in  many  minds,  because,  for  the  most  part, 
masses  were  comparatively  estimated  by  the  inter- 
mediary of  weights.  The  estimations  of  weight  made 
with  the  balance  utilize  the  action  of  the  weight  on 
the  beam,  but  in  such  conditions  that  the  influence  of 


32     THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

the  variations  of  gravity  becomes  eliminated.  The 
two  weights  which  are  being  compared  may  both  of 
them  change  if  the  weighing  is  effected  in  different 
places,  but  they  are  attracted  in  the  same  proportion. 
If  once  equal,  they  remain  equal  even  when  in 
reality  they  may  both  have  varied. 

The  current  law  defines  the  kilogramme  as  the 
standard  of  mass,  and  the  law  is  certainly  in 
conformity  with  the  rather  obscurely  expressed 
intentions  of  the  founders  of  the  metrical  system. 
Their  terminology  was  vague,  but  they  certainly 
had  in  view  the  supply  of  a  standard  for  commercial 
transactions,  and  it  is  quite  evident  that  in  barter 
what  is  important  to  the  buyer  as  well  as  to  the 
seller  is  not  the  attraction  the  earth  may  exercise  on 
the  goods,  but  the  quantity  that  may  be  supplied  for 
a  given  price.  Besides,  the  fact  that  the  founders 
abstained  from  indicating  any  specified  spot  in  the 
definition  of  the  kilogramme,  when  they  were  per- 
fectly acquainted  with  the  considerable  variations 
in  the  intensity  of  gravity,  leaves  no  doubt  as  to 
their  real  desire. 

The  same  objections  have  been  made  to  the 
definition  of  the  kilogramme,  at  first  considered  as 
the  mass  of  a  cubic  decimetre  of  water  at  4°  C.,  as  to 
the  first  definition  of  the  metre.  We  must  admire 
the  incredible  precision  attained  at  the  outset  by  the 
physicists  who  made  the  initial  determinations,  but 
we  know  at  the  present  day  that  the  kilogramme 


MEASUREMENTS  33 

they  constructed  is  slightly  too  heavy  (by  about 
25,o"oo)-  Very  remarkable  researches  have  been 
carried  out  with  regard  to  this  determination  by 
the  International  Bureau,  and  by  MM.  Mace  de 
Lepinay  and  Buisson.  The  law  of  the  llth  July 
1903  has  definitely  regularized  the  custom  which 
physicists  had  adopted  some  years  before;  and 
the  standard  of  mass,  the  legal  prototype  of  the 
metrical  system,  is  now  the  international  kilogramme 
sanctioned  by  the  Conference  of  Weights  and 
Measures. 

The  comparison  of  a  mass  with  the  standard 
is  effected  with  a  precision  to  which  no  other 
measurement  can  attain.  Metrology  vouches  for 
the  hundredth  of  a  milligramme  in  a  kilogramme  ; 
that  is  to  say,  that  it  estimates  the  hundred-millionth 
part  of  the  magnitude  studied. 

We  may — as  in  the  case  of  the  lengths — ask  our- 
selves whether  this  already  admirable  precision  can 
be  surpassed  ;  and  progress  would  seem  likely  to 
be  slow,  for  difficulties  singularly  increase  when  we 
get  to  such  small  quantities.  But  it  is  permitted 
to  hope  that  the  physicists  of  the  future  will  do  still 
better  than  those  of  to-day;  and  perhaps  we  may 
catch  a  glimpse  of  the  time  when  we  shall  begin  to 
observe  that  the  standard,  which  is  constructed  from 
a  heavy  metal,  namely,  iridium-platinum,  itself  obeys 
an  apparently  general  law,  and  little  by  little  loses 
some  particles  of  its  mass  by  emanation. 

3 


34    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

§  4.  THE  MEASURE  OF  TIME 

The  third  fundamental  magnitude  of  mechanics  is 
time.  There  is,  so  to  speak,  no  physical  phenomenon 
in  which  the  notion  of  time  linked  to  the  sequence 
of  our  states  of  consciousness  does  not  play  a  con- 
siderable part. 

Ancestral  habits  and  a  very  early  tradition  have 
led  us  to  preserve,  as  the  unit  of  time,  a  unit  con- 
nected with  the  earth's  movement;  and  the  unit 
to-day  adopted  is,  as  we  know,  the  sexagesimal 
second  of  mean  time.  This  magnitude,  thus  defined 
by  the  conditions  of  a  natural  motion  which  may 
itself  be  modified,  does  not  seem  to  offer  all  the 
guarantees  desirable  from  the  point  of  view  of 
invariability.  It  is  certain  that  all  the  friction  exer- 
cised on  the  earth — by  the  tides,  for  instance — must 
slowly  lengthen  the  duration  of  the  day,  and  must 
influence  the  movement  of  the  earth  round  the  sun. 
Such  influence  is  certainly  very  slight,  but  it  never- 
theless gives  an  unfortunately  arbitrary  character 
to  the  unit  adopted. 

We  might  have  taken  as  the  standard  of  time 
the  duration  of  another  natural  phenomenon,  which 
appears  to  be  always  reproduced  under  identical 
conditions ;  the  duration,  for  instance,  of  a  given 
luminous  vibration.  But  the  experimental  diffi- 
culties of  evaluation  ^with  such  a  unit  of  the 
times  which  ordinarilv  have  to  be  considered,  would 


MEASUREMENTS  35 

be  so  great  that  such  a  reform  in  practice  cannot 
be  hoped  for.  It  should,  moreover,  be  remarked 
that  the  duration  of  a  vibration  may  itself  be  in- 
fluenced by  external  circumstances,  among  which 
are  the  variations  of  the  magnetic  field  in  which 
its  source  is  placed.  It  could  not,  therefore,  be 
strictly  considered  as  independent  of  the  earth; 
and  the  theoretical  advantage  which  might  be 
expected  from  this  alteration  would  be  somewhat 
illusory. 

Perhaps  in  the  future  recourse  may  be  had  to  very 
different  phenomena.  Thus  Curie  pointed  out  that 
if  the  air  inside  a  glass  tube  has  been  rendered 
radioactive  by  a  solution  of  radium,  the  tube  may 
be  sealed  up,  and  it  will  then  be  noted  that  the 
radiation  of  its  walls  diminishes  with  time,  in 
accordance  with  an  exponential  law.  The  constant 
of  time  defined  by  this  phenomenon  remains  the 
same  whatever  the  nature  and  dimensions  of  the 
walls  of  the  tube  or  the  temperature  may  be,  and 
time  might  thus  be  defined  independently  of  all  the 
other  units. 

We  might  also,  as  M.  Lippmann  has  suggested  in  an 
extremely  ingenious  way,  decide  to  obtain  measures 
of  time  which  can  be  considered  as  absolute  because 
they  are  determined  by  parameters  of  another  nature 
than  that  of  the  magnitude  to  be  measured.  Such 
experiments  are  made  possible  by  the  phenomena 
of  gravitation.  We  could  employ,  for  instance,  the 


36    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

pendulum  by  adopting,  as  the  unit  of  force,  the 
force  which  renders  the  constant  of  gravitation 
equal  to  unity.  The  unit  of  time  thus  defined 
would  be  independent  of  the  unit  of  length,  and 
would  depend  only  on  the  substance  which  would 
give  us  the  unit  of  mass  under  the  unit  of  volume. 

It  would  be  equally  possible  to  utilize  electrical 
phenomena,  and  one  might  devise  experiments  per- 
fectly easy  of  execution.  Thus,  by  charging  a  con- 
denser by  means  of  a  battery,  and  discharging  it  a 
given  number  of  times  in  a  given  interval  of  time, 
so  that  the  effect  of  the  current  of  discharge  should 
be  the  same  as  the  effect  of  the  output  of  the 
battery  through  a  given  resistance,  we  could  estimate, 
by  the  measurement  of  the  electrical  magnitudes,  the 
duration  of  the  interval  noted.  A  system  of  this 
kind  must  not  be  looked  upon  as  a  simple  jeu  dy  esprit, 
since  this  very  practicable  experiment  would  easily 
permit  us  to  check,  with  a  precision  which  could 
be  carried  very  far,  the  constancy  of  an  interval 
of  time. 

From  the  practical  point  of  view,  chronometry 
has  made  in  these  last  few  years  very  sensible 
progress.  The  errors  in  the  movements  of  chrono- 
meters are  corrected  in  a  much  more  systematic  way 
than  formerly,  and  certain  inventions  have  enabled 
important  improvements  to  be  effected  in  the  con- 
struction of  these  instruments.  Thus  the  curious 
properties  which  steel  combined  with  nickel — so 


MEASUREMENTS  37 

admirably  studied  by  M.  Ch.  Ed.  Guillaume — ex- 
hibits in  the  matter  of  dilatation  are  now  utilized 
so  as  to  almost  completely  annihilate  the  influence 
of  variations  of  temperature. 

§  5.  THE  MEASURE  OF  TEMPERATURE 

From  the  three  mechanical  units  we  derive 
secondary  units ;  as,  for  instance,  the  unit  of  work  or 
mechanical  energy.  The  kinetic  theory  takes  tem- 
perature, as  well  as  heat  itself,  to  be  a  quantity  of 
energy,  and  thus  seems  to  connect  this  notion  with 
the  magnitudes  of  mechanics.  But  the  legitimacy 
of  this  theory  cannot  be  admitted,  and  the  calorific 
movement  should  also  be  a  phenomenon  so  strictly 
confined  in  space  that  our  most  delicate  means  of 
investigation  would  not  enable  us  to  perceive  it.  It 
is  better,  then,  to  continue  to  regard  the  unit  of 
difference  of  temperature  as  a  distinct  unit,  to  be 
added  to  the  fundamental  units. 

To  define  the  measure  of  a  certain  temperature, 
we  take,  in  practice,  some  arbitrary  property  of  a 
body.  The  only  necessary  condition  of  this  property 
is,  that  it  should  constantly  vary  in  the  same  direc- 
tion when  the  temperature  rises,  and  that  it  should 
possess,  at  any  temperature,  a  well-marked  value. 
We  measure  this  value  by  melting  ice  and  by  the 
vapour  of  boiling  water  under  normal  pressure,  and 
the  successive  hundred ths  of  its  variation,  beginning 
with  the  melting  ice,  defines  the  percentage. 


38    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

Thermodynamics,  however,  has  made  it  plain  that 
we  can  set  up  a  thermometric  scale  without  relying 
upon  any  determined  property  of  a  real  body.  Such 
a  scale  has  an  absolute  value  independently  of 
the  properties  of  matter.  Now  it  happens  that  if 
we  make  use  for  the  estimation  of  temperatures,  of 
the  phenomena  of  dilatation  under  a  constant  pres- 
sure, or  of  the  increase  of  pressure  in  a  constant 
volume  of  a  gaseous  body,  we  obtain  a  scale  very 
near  the  absolute,  which  almost  coincides  with  it 
when  the  gas  possesses  certain  qualities  which  make 
it  nearly  what  is  called  a  perfect  gas.  This  most 
lucky  coincidence  has  decided  the  choice  of  the  con- 
vention adopted  by  physicists.  They  define  normal 
temperature  by  means  of  the  variations  of  pressure 
in  a  mass  of  hydrogen  beginning  with  the  initial 
pressure  of  a  metre  of  mercury  at  0°  C. 

M.  P.  Chappuis,  in  some  very  precise  experiments 
conducted  with  much  method,  has  proved  that  at 
ordinary  temperatures  the  indications  of  such  a 
thermometer  are  so  close  to  the  degrees  of  the 
theoretical  scale  that  it  is  almost  impossible  to 
ascertain  the  value  of  the  divergences,  or  even  the 
direction  that  they  take.  The  divergence  becomes, 
however,  manifest  when  we  work  with  extreme 
temperatures.  It  results  from  the  useful  researches 
of  M.  Daniel  Berthelot  that  we  must  subtract 
-fO'180  from  the  indications  of  the  hydrogen  ther- 
mometer towards  the  temperature  —240°  C.,  and  add 


MEASUREMENTS  39 

+  0'05°  to  1000°  to  equate  them  with  the  thermo- 
dynamic  scale.  Of  course,  the  difference  would  also 
become  still  more  noticeable  on  getting  nearer  to 
the  absolute  zero ;  for  as  hydrogen  gets  more  and 
more  cooled,  it  gradually  exhibits  in  a  lesser  degree 
the  characteristics  of  a  perfect  gas. 

To  study  the  lower  regions  which  border  on  that 
kind  of  pole  of  cold  towards  which  are  straining 
the  efforts  of  the  many  physicists  who  have  of  late 
years  succeeded  in  getting  a  few  degrees  further 
forward,  we  may  turn  to  a  gas  still  more  difficult 
to  liquefy  than  hydrogen.  Thus,  thermometers 
have  been  made  of  helium;  and  from  the  tem- 
perature of  —260°  C.  downward  the  divergence  of 
such  a  thermometer  from  one  of  hydrogen  is 
very  marked. 

The  measurement  of  very  high  temperatures  is  not 
open  to  the  same  theoretical  objections  as  that  of 
very  low  temperatures ;  but,  from  a  practical  point 
of  view,  it  is  as  difficult  to  effect  with  an  ordinary 
gas  thermometer.  It  becomes  impossible  to  guaran- 
tee the  reservoir  remaining  sufficiently  impermeable, 
and  all  security  disappears,  notwithstanding  the  use 
of  recipients  very  superior  to  those  of  former  times, 
such  as  those  lately  devised  by  the  physicists  of  the 
Reichansalt.  This  difficulty  is  obviated  by  using 
other  methods,  such  as  the  employment  of  thermo- 
electric couples,  such  as  the  very  convenient  couple 
of  M.  le  Chatelier ;  but  the  graduation  of  these  in- 


40    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

struments  can  only  be  effected  at  the  cost  of  'a  rather 
bold  extrapolation.. 

M.  D.  Berthelot  has  pointed  out  and  experimented 
with  a  very  interesting  process,  founded  on  the 
measurement  by  the  phenomena  of  interference  of 
the  refractive  index  of  a  column  of  air  subjected  to 
the  temperature  it  is  desired  to  measure.  It  appears 
admissible  that  even  at  the  highest  temperatures  the 
variation  of  the  power  of  refraction  is  strictly  pro- 
portional to  that  of  the  density,  for  this  proportion 
is  exactly  verified  so  long  as  it  is  possible  to  check 
it  precisely.  We  can  thus,  by  a  method  which 
offers  the  great  advantage  of  being  independent  of 
the  power  and  dimension  of  the  envelopes  employed 
— since  the  length  of  the  column  of  air  considered  alone 
enters  into  the  calculation — obtain  results  equivalent 
to  those  given  by  the  ordinary  air  thermometer. 

Another  method,  very  old  in  principle,  has  also 
lately  acquired  great  importance.  For  a  long  time 
we  sought  to  estimate  the  temperature  of  a  body  by 
studying  its  radiation,  but  we  did  not  know  any  posi- 
tive relation  between  this  radiation  and  the  tempera- 
ture, and  we  had  no  good  experimental  method  of 
estimation,  but  had  recourse  to  purely  empirical 
formulas  and  the  use  of  apparatus  of  little  precision. 
Now,  however,  many  physicists,  continuing  the  classic 
researches  of  Kirchhoff,  Boltzmann,  Professors  Wien 
and  Planck,  and  taking  their  starting-point  from  the 
laws  of  thermodynamics,  have  given  formulas  which 


MEASUREMENTS  41 

establish  the  radiating  power  of  a  dark  body  as  a  func- 
tion of  the  temperature  and  the  wave-length,  or,  better 
•  still,  of  the  total  power  as  a  function  of  the  tempera- 
ture and  wave-length  corresponding  to  the  maximum 
value  of  the  power  of  radiation.  We  see,  therefore, 
the  possibility  of  appealing  for  the  measurement  of 
temperature  to  a  phenomenon  which  is  no  longer  the 
variation  of  the  elastic  force  of  a  gas,  and  yet  is  also 
connected  with  the  principles  of  thermodynamics. 

This  is  what  Professors  Lummer  and  Pringsheim 
have  shown  in  a  series  of  studies  which  may  cer- 
tainly be  reckoned  among  the  greatest  experimental 
researches  of  the  last  few  years.  They  have  con- 
structed a  radiator  closely  resembling  the  theoreti- 
cally integral  radiator  which  a  closed  isothermal 
vessel  would  be,  and  with  only  a  very  small  opening, 
which  allows  us  to  collect  from  outside  the  radiations 
which  are  in  equilibrium  with  the  interior.  This 
vessel  is  formed  of  a  hollow  carbon  cylinder,  heated 
by  a  current  of  high  intensity;  the  radiations  are 
studied  by  means  of  a  bolometer,  the  disposition  of 
which  varies  with  the  nature  of  the  experiments. 

It  is  hardly  possible  to  enter  into  the  details  of 
the  method,  but  the  result  sufficiently  indicates  its 
importance.  It  is  now  possible,  thanks  to  their 
researches,  to  estimate  a  temperature  of  2000°  C. 
to  within  about  5°.  Ten  years  ago  a  similar  ap- 
proximation could  hardly  have  been  arrived  at  for 
a  temperature  of  1000°  C. 


42     THE  NEW   PHYSICS  AND   ITS  EVOLUTION 

§  6.  DERIVED  UNITS  AND  THE  MEASURE  OF 
A  QUANTITY  OF  ENERGY 

It  must  be  understood  that  it  is  only  by  arbitrary 
convention  that  a  dependency  is  established  between 
a  derived  unit  and  the  fundamental  units.  The 
laws  of  numbers  in  physics  are  often  only  laws 
of  proportion.  We  transform  them  into  laws 
of  equation,  because  we  introduce  numerical  co- 
efficients and  choose  the  units  on  which  they  depend 
so  as  to  simplify  as  much  as  possible  the  formulas 
most  in  use.  A  particular  speed,  for  instance,  is  in 
reality  nothing  else  but  a  speed,  and  it  is  only  by 
the  peculiar  choice  of  unit  that  we  can  say  that  it 
is  the  space  covered  during  the  unit  of  time.  In 
the  same  way,  a  quantity  of  electricity  is  a  quantity 
of  electricity  ;  and  there  is  nothing  to  prove  that, 
in  its  essence,  it  is  really  reducible  to  a  function  of 
mass,  of  length,  and  of  time. 

Persons  are  still  to  be  met  with  who  seem  to 
have  some  illusions  on  this  point,  and  who  see  in  the 
doctrine  of  the  dimensions  of  the  units  a  doctrine  of 
general  physics,  while  it  is,  to  say  truth,  only  a  doc- 
trine of  metrology.  The  knowledge  of  dimensions 
is  valuable,  since  it  allows  us,  for  instance,  to  easily 
verify  the  homogeneity  of  a  formula,  but  it  can  in 
no  way  give  us  any  information  on  the  actual  nature 
of  the  quantity  measured. 

Magnitudes  to  which  we  attribute  like  dimensions 


MEASUREMENTS  %      43 

may  be  qualitatively  irreducible  one  to  the  other. 
Thus  the  different  forms  of  energy  are  measured  by 
the  same  unit,  and  yet  it  seems  that  some  of  them, 
such  as  kinetic  energy,  really  depend  on  time  ;  while 
for  others,  such  as  potential  energy,  the  dependency 
established  by  the  system  of  measurement  seems 
somewhat  fictitious. 

The  numerical  value  of  a  quantity  of  energy  of 
any  nature  should,  in  the  system  C.G-.S.,  be  ex- 
pressed in  terms  of  the  unit  called  the  erg ;  but,  as 
a  matter  of  fact,  when  we  wish  to  compare  and 
measure  different  quantities  of  energy  of  varying 
forms,  such  as  electrical,  chemical,  and  other  quanti- 
ties, etc.,  we  nearly  always  employ  a  method  by 
which  all  these  energies  are  finally  transformed  and 
used  to  heat  the  water  of  a  calorimeter.  It  is  there- 
fore very  important  to  study  well  the  calorific 
phenomenon  chosen  as  the  unit  of  heat,  and  to  deter- 
mine with  precision  its  mechanical  equivalent,  that 
is  to  say,  the  number  of  ergs  necessary  to  produce 
this  unit.  This  is  a  number  which,  on  the  principle 
of  equivalence,  depends  neither  on  the  method 
employed,  nor  the  time,  nor  any  other  external 
circumstance. 

As  the  result  of  the  brilliant  researches  of  Rowland 
and  of  Mr  Griffiths  on  the  variations  of  the  specific 
heat  of  water,  physicists  have  decided  to  take  as 
calorific  standard  the  quantity  of  heat  necessary  to 
raise  a  gramme  of  water  from  15°  to  16°  C.,  the 


44    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

temperature  being  measured  by  the  scale  of  the 
hydrogen  thermometer  of  the  International  Bureau. 

On  the  other  hand,  new  determinations  of  the 
mechanical  equivalent,  among  which  it  is  right  to 
mention  that  of  Mr  Ames,  and  a  full  discussion 
as  to  the  best  results,  have  led  to  the  adoption  of 
the  number  4-187  to  represent  the  number  of  ergs 
capable  of  producing  the  unit  of  heat. 

In  practice,  the  measurement  of  a  quantity  of 
heat  is  very  often  effected  by  means  of  the  ice 
calorimeter,  the  use  of  which  is  particularly  simple 
and  convenient.  There  is,  therefore,  a  very  special 
interest  in  knowing  exactly  the  melting-point  of 
ice.  M.  Leduc,  who  for  several  years  has  measured 
a  great  number  of  physical  constants  with  minute 
precautions  and  a  remarkable  sense  o£  precision, 
concludes,  after  a  close  discussion  of  the  various 
results  obtained,  that  this  heat  is  equal  to  791 
calories.  An  error  of  almost  a  calorie  had  been 
committed  by  several  renowned  experimenters,  and 
it  will  be  seen  that  in  certain  points  the  art  of 
measurement  may  still  be  largely  perfected. 

To  the  unit  of  energy  might  be  immediately 
attached  other  units.  For  instance,  radiation  being 
nothing  but  a  flux  of  energy,  we  could,  in  order  to 
establish  photometric  units,  divide  the  normal 
spectrum  into  bands  of  a  given  width,  and  measure 
the  power  of  each  for  the  unit  of  radiating  surface. 

But,  notwithstanding  some   recent  researches  on 


MEASUREMENTS  45 

this  question,  we  cannot  yet  consider  the  distribu- 
tion of  energy  in  the  spectrum  as  perfectly  known. 
If  we  adopt  the  excellent  habit  which  exists  in  some 
researches  of  expressing  radiating  energy  in  ergs, 
it  is  still  customary  to  bring  the  radiations  to  a 
standard  giving,  by  its  constitution  alone,  the  unit 
of  one  particular  radiation.  In  particular,  the  de- 
finitions are  still  adhered  to  which  were  adopted 
as  the  result  of  the  researches  of  M.  Violle  on 
the  radiation  of  fused  platinum  at  the  tempera- 
ture of  solidification  ;  and  most  physicists  utilize  in 
the  ordinary  methods  of  photometry  the  clearly 
defined  notions  of  M.  Blondel  as  to  the  luminous 
intensity  of  flux,  illumination  (falairement),  light 
(6dat\  and  lighting  (tclairage),  with  the  correspond- 
ing units,  decimal  candle,  lumen,  liix,  carcel  lamp, 
candle  per  square  centimetre,  and  lumen-hour.1 

§  7.  MEASURE  OF  CERTAIN  PHYSICAL 
CONSTANTS 

The  progress  of  metrology  has  led,  as  a  conse- 
quence, to  corresponding  progress  in  nearly  all  physi- 
cal measurements,  and  particularly  in  the  measure 
of  natural  constants.  Among  these,  the  constant  of 
gravitation  occupies  a  position  quite  apart  from  the 
importance  and  simplicity  of  the  physical  law  which 

1  These  are  the  magnitudes  and  units  adopted  at  the  Inter- 
national Congress  of  Electricians  in  1904.  For  their  definition 
and  explanation,  see  Demanet,  Notes  de  Physique  Experimentale 
(Louvain,  1905),  t.  iv.  p.  8.— ED, 


46    THE   NEW   PHYSICS  AND   ITS   EVOLUTION 

defines  it,  as  well  as  by  its  generality.  Two  material 
particles  are  mutually  attracted  to  each  other  by  a 
force  directly  proportional  to  the  product  of  their 
mass,  and  inversely  proportional  to  the  square  of  the 
distance  between  them.  The  coefficient  of  propor- 
tion is  determined  when  once  the  units  are  chosen, 
and  as  soon  as  we  know  the  numerical  values  of 
this  force,  of  the  two  masses,  and  of  their  distance. 
But  when  we  wish  to  make  laboratory  experiments 
serious  difficulties  appear,  owing  to  the  weakness  of 
the  attraction  between  masses  of  ordinary  dimen- 
sions. Microscopic  forces,  so  to  speak,  have  to  be 
observed,  and  therefore  all  the  causes  of  errors  have 
to  be  avoided  which  would  be  unimportant  in  most 
other  physical  researches.  It  is  known  that  Caven- 
dish was  the  first  who  succeeded  by  means  of  the 
torsion  balance  in  effecting  fairly  precise  measure- 
ments. This  method  has  been  again  taken  in  hand  by 
different  experimenters,  and  the  most  recent  results 
are  due  to  Mr  Yernon  Boys.  This  learned  physicist 
is  also  the  author  of  a  most  useful  practical  inven- 
tion, and  has  succeeded  in  making  quartz  threads 
as  fine  as  can  be  desired  and  extremely  uniform. 
He  finds  that  these  threads  possess  valuable  pro- 
perties, such  as  perfect  elasticity  and  great  tenacity. 
He  has  been  able,  with  threads  not  more  than  ^J0  of 
a  millimetre  in  diameter,  to  measure  with  precision 
couples  of  an  order  formerly  considered  outside  the 
range  of  experiment,  and  to  reduce  the  dimensions 


MEASUREMENTS  47 

of  the  apparatus  of  Cavendish  in  the  proportion  of 
150  to  1.  The  great  advantage  found  in  the  use 
of  Ihese  small  instruments  is  the  better  avoidance 
of  the  perturbations  arising  from  draughts  of  air, 
and  of  the  very  serious  influence  of  the  slightest 
inequality  in  temperature. 

Other  methods  have  been  employed  in  late  years 
by  other  experimenters,  such  as  the  method  of 
Baron  Eotvos,  founded  on  the  use  of  a  torsion  lever, 
the  method  of  the  ordinary  balance,  used  especially 
by  Professors  Eicharz  and  Krigar-Menzel  and  also  by 
Professor  Poynting,  and  the  method  of  M.  Wilsing, 
who  uses  a  balance  with  a  vertical  beam.  The 
results  fairly  agree,  and  lead  to  attributing  to  the 
earth  a  density  equal  to  5 '5 27. 

The  most  familiar  manifestation  of  gravitation  is 
gravity.  The  action  of  the  earth  on  the  unit  of  mass 
placed  in  one  point,  and  the  intensity  of  gravity,  is 
measured,  as  we  know,  by  the  aid  of  a  pendulum. 
The  methods  of  measurement,  whether  by  absolute 
or  by  relative  determinations,  so  greatly  improved 
by  Borda  and  Bessel,  have  been  still  further  im- 
proved by  various  geodesians,  among  whom  should 
be  mentioned  M.  von  Sterneek  and  General 
Defforges.  Numerous  observations  have  been  made 
in  all  parts  of  the  world  by  various  explorers,  and 
have  led  to  a  fairly  complete  knowledge  of  the 
distribution  of  gravity  over  the  surface  of  the  globe. 
Thus  we  have  succeeded  in  making  evident  anoma- 


48    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

lies  which  would  not  easily  find  their  place  in  the 
formula  of  Clairaut. 

Another  constant,  the  determination  of  which  is 
of  the  greatest  utility  in  astronomy  of  position,  and 
the  value  of  which  enters  into  electromagnetic 
theory,  has  to-day  assumed,  with  the  new  ideas  on  the 
constitution  of  matter,  a  still  more  considerable 
importance.  I  refer  to  the  speed  of  light,  which 
appears  to  us,  as  we  shall  see  further  on,  the 
maximum  value  of  speed  which  can  be  given  to  a 
material  body. 

After  the  historical  experiments  of  Fizeau  and 
Foucault,  taken  up  afresh,  as  we  know,  partly  by 
Cornu,  and  partly  by  Michelson  and  Newcomb,  it 
remained  still  possible  to  increase  the  precision  of  the 
measurements.  Professor  Michelson  has  undertaken 
some  new  researches  by  a  method  which  is  a  com- 
bination of  the  principle  of  the  toothed  wheel  of 
Fizeau  with  the  revolving  mirror  of  Foucault.  The 
toothed  wheel  is  here  replaced,  however,  by  a  grating, 
in  which  the  lines  and  the  spaces  between  them  take 
the  place  of  the  teeth  and  the  gaps,  the  reflected 
light  only  being  returned  when  it  strikes  on  the 
space  between  two  lines.  The  illustrious  American 
physicist  estimates  that  he  can  thus  evaluate  to 
nearly  five  kilometres  the  path  traversed  by  light  in 
one  second.  This  approximation  corresponds  to  a 
relative  value  of  a  few  hundred-thousandths,  and 
it  far  exceeds  those  hitherto  attained  by  the  best 


MEASUREMENTS  49 

experimenters.  When  all  the  experiments  are 
completed,  they  will  perhaps  solve  certain  questions 
still  in  suspense ;  for  instance,  the  question  whether 
the  speed  of  propagation  depends  on  intensity.  If 
this  turns  out  to  be  the  case,  we  should  be  brought 
to  the  important  conclusion  that  the  amplitude 
of  the  oscillations,  which  is  certainly  very  small 
in  relation  to  the  already  tiny  wave-lengths,  cannot 
be  considered  as  unimportant  in  .regard  to  these 
lengths.  Such  would  seem  to  have  been  the  result 
of  the  curious  experiments  of  M.  Muller  and  of 
M.  Ebert,  but  these  results  have  been  recently 
disputed  by  M.  Doubt. 

In  the  case  of  sound  vibrations,  on  the  other 
hand,  it  should  be  noted  that  experiment,  consistently 
with  the  theory,  proves  that  the  speed  increases 
with  the  amplitude,  or,  if  you  will,  with  the  intensity. 
M.  Violle  has  published  an  important  series  of 
experiments  on  the  speed  of  propagation  of  very 
condensed  waves,  on  the  deformations  of  these  waves, 
and  on  the  relations  of  the  speed  and  the  pressure, 
which  verify  in  a  remarkable  manner  the  results 
foreshadowed  by  the  already  old  calculations  of 
Riemann,  repeated  later  by  Hugoniot.  If,  on  the 
contrary,  the  amplitude  is  sufficiently  small,  there 
exists  a  speed  limit  which  is  the  same  in  a  large  pipe 
and  in  free  air.  By  some  beautiful  experiments, 
MM.  Violle  and  Vautier  have  clearly  shown 
that  any  disturbance  in  the  air  melts  somewhat 

4 


50    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

quickly  into  a  single  wave  of  given  form,  which 
is  propagated  to  a  distance,  while  gradually  becoming 
weaker  and  showing  a  constant  speed  which  differs 
little  in  dry  air  at  0°  C.  from  331*36  metres  per 
second.  In  a  narrow  pipe  the  influence  of  the 
walls  makes  itself  felt  and  produces  various  effects, 
in  particular  a  kind  of  dispersion  in  space  of  the 
harmonics  of  the  sound.  This  phenomenon,  according 
to  M.  Brillouin,  is  perfectly  explicable  by  a  theory 
similar  to  the  theory  of  gratings. 


CHAPTEE  III 
PRINCIPLES 

§  1:  THE  PRINCIPLES  OF  PHYSICS 

FACTS  conscientiously  observed  lead  by  induction 
to  the  enunciation  of  a  certain  number  of  laws  or 
general  hypotheses  which  are  the  principles  already 
referred  to.  These  principal  hypotheses  are,  in  the 
eyes  of  a  physicist,  legitimate  generalizations,  the 
consequences  of  which  we  shall  be  able  at  once  to 
check  by  the  experiments  from  which  they  issue. 

Among  the  principles  almost  universally  adopted 
until  lately  figure  prominently  those  of  mechanics — 
such  as  the  principle  of  relativity,  and  the  principle 
of  the  equality  of  action  and  reaction.  We  will  not 
detail  nor  discuss  them  here,  but  later  on  we  shall 
have  an  opportunity  of  pointing  out  how  recent 
theories  on  the  phenomena  of  electricity  have  shaken 
the  confidence  of  physicists  in  them  and  have  led 
certain  scholars  to  doubt  their  absolute  value. 

The  principle  of  Lavoisier,  or  principle  of  the  con- 
servation of  mass,  presents  itself  under  two  different 


52     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

aspects  according  to  whether  mass  is  looked  upon  as 
the  coefficient  of  the  inertia  of  matter  or  as  the 
factor  which  intervenes  in  the  phenomena  of  uni- 
versal attraction,  and  particularly  in  gravitation. 
We  shall  see  when  we  treat  of  these  theories,  how  we 
have  been  led  to  suppose  that  inertia  depended  on 
velocity  and  even  on  direction.  If  this  conception 
were  exact,  the  principle  of  the  invariability  of  mass 
would  naturally  be  destroyed.  Considered  as  a 
factor  of  attraction,  is  mass  really  indestructible  ? 

A  few  years  ago  such  a  question  would  have  seemed 
singularly  audacious.  And  yet  the  law  of  Lavoisier  is 
so  far  from  self-evident  that  for  centuries  it  escaped 
the  notice  of  physicists  and  chemists.  But  its  great 
apparent  simplicity  and  its  high  character  of  gener- 
ality, when  enunciated  at  the  end  of  the  eighteenth 
century,  rapidly  gave  it  such  an  authority  that  no 
one  was  able  to  any  longer  dispute  it  unless  he 
desired  the  reputation  of  an  oddity  inclined  to 
paradoxical  ideas. 

It  is  important,  however,  to  remark  that,  under 
fallacious  metaphysical  appearances,  we  are  in  reality 
using  empty  words  when  we  repeat  the  aphorism, 
"  Nothing  can  be  lost,  nothing  can  be  created,"  and 
deduce  from  it  the  indestructibility  of  matter. 
This  indestructibility,  in  truth,  is  an  experimental 
fact,  and  the  principle  depends  on  experiment.  It 
may  even  seem,  at  first  sight,  more  singular  than  not 
that  the  weight  of  a  bodily  system  in  a  given  place, 


PRINCIPLES  53 

or  the  quotient  of  this  weight  by  that  of  the  standard 
mass — that  is  to  say,  the  mass  of  these  bodies — remains 
invariable,  both  when  the  temperature  changes  and 
when  chemical  reagents  cause  the  original  materials 
to-  disappear  and  to  be  replaced  by  new  ones.  We 
may  certainly  consider  that  in  a  chemical  pheno- 
menon annihilations  and  creations  of  matter  are 
really  produced;  but  the  experimental  law  teaches 
us  that  there  is  compensation  in  certain  respects. 

The  discovery  of  the  radioactive  bodies  has,  in 
some  sort,  rendered  popular  the  speculations  of 
physicists  on  the  phenomena  of  the  disaggregation 
of  matter.  We  shall  have  to  seek  the  exact  meaning 
which  ought  to  be  given  to  the  experiments  on  the 
emanation  of  these  bodies,  and  to  discover  whether 
these  experiments  really  imperil  the  law  of  Lavoisier. 

For  some  years  different  experimenters  have  also 
effected  many  very  precise  measurements  of  the 
weight  of  divers  bodies  both  before  and  after  chemical 
reactions  between  these  bodies.  Two  highly  experi- 
enced and  cautious  physicists,  Professors  Landolt  and 
Heydweiller,  have  not  hesitated  to  announce  the 
sensational  result  that  in  certain  circumstances  the 
weight  is  no  longer  the  same  after  as  before  the 
reaction.  In  particular,  the  weight  of  a  solution 
of  salts  of  copper  in  water  is  not  the  exact  sum 
of  the  joint  weights  of  the  salt  and  the  water. 
Such  experiments  are  evidently  very  delicate ; 
they  have  been  disputed,  and  they  cannot  be  con- 


54     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

sidered  as  sufficient  for  conviction.  It  follows 
nevertheless  that  it  is  no  longer  forbidden  to  regard 
the  law  of  Lavoisier  as  only  an  approximate  law ; 
according  to  Sandford  and  Kay,  this  approximation 

would  be  about  ^,400,^00-  ^^s  *s  a^so  ^ne  result 
reached  by  Professor  Poynting  in  experiments  regard- 
ing the  possible  action  of  temperature  on  the  weight 
of  a  body ;  and  if  this  be  really  so,  we  may  reassure 
ourselves,  and  from  the  point  of  view  of  practical 
application  may  continue  to  look  upon  matter  as 
indestructible. 

The  principles  of  physics,  by  imposing  certain 
conditions  on  phenomena,  limit  after  a  fashion  the 
field  of  the  possible.  Among  these  principles  is  one 
which,  notwithstanding  its  importance  when  com- 
pared with  that  of  universally  known  principles, 
is  less  familiar  to  some  people.  This  is  the  principle 
of  symmetry,  more  or  less  conscious  applications  of 
which  can,  no  doubt,  be  found  in  various  works  and 
even  in  the  conceptions  of  Copernican  astronomers, 
but  which  was  generalized  and  clearly  enunciated 
for  the  first  time  by  the  late  M.  Curie.  This 
illustrious  physicist  pointed  out  the  advantage  of 
introducing  into  the  study  of  physical  phenomena 
the  considerations  on  symmetry  familiar  to  crystal- 
lographers ;  for  a  phenomenon  to  take  place,  it  is 
necessary  that  a  certain  dissymmetry  should  previ- 
ously exist  in  the  medium  in  which  this  phenomenon 
occurs.  A  body,  for  instance,  may  be  animated  with 


PRINCIPLES  55 

a  certain  linear  velocity  or  a  speed  of  rotation ;  it 
may  be  compressed,  or  twisted ;  it  may  be  placed  in 
an  electric  or  in  a  magnetic  field ;  it  may  be  affected 
by  an  electric  current  or  by  one  of  heat;  it  may 
be  traversed  by  a  ray  of  light  either  ordinary  or 
polarized  rectilineally  or  circularly,  etc. : — in  each 
case  a  certain  minimum  and  characteristic  dis- 
symmetry is  necessary  at  every  point  of  the 
body  in  question. 

This  consideration  enables  us  to  foresee  that 
certain  phenomena  which  might  be  imagined  a 
priori  cannot  exist.  Thus,  for  instance,  it  is  im- 
possible that  an  electric  field,  a  magnitude  directed 
and  not  superposable  on  its  image  in  a  mirror 
perpendicular  to  its  direction,  could  be  created  at 
right  angles  to  the  plane  of  symmetry  of  the 
medium;  while  it  would  be  possible  to  create  a 
magnetic  field  under  the  same  conditions. 

This  consideration  thus  leads  us  to  the  discovery 
of  new  phenomena  ;  but  it  must  be  understood  that 
it  cannot  of  itself  give  us  absolutely  precise  notions 
as  to  the  nature  of  these  phenomena,  nor  disclose 
their  order  of  magnitude. 

§  2.  THE  PRINCIPLE  OF  THE  CONSERVATION 
OF  ENERGY 

Dominating  not  physics  alone,  but  nearly  every 
other  science,  the  principle  of  the  conservation  of 
energy  is  justly  considered  as  the  grandest  conquest 


56     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

of  contemporary  thought.  It  shows  us  in  a  powerful 
light  the  most  diverse  questions  ;  it  introduces  order 
into  the  most  varied  studies ;  it  leads  to  a  clear  and 
coherent  interpretation  of  phenomena  which,  with- 
out it,  appear  to  have  no  connexion  with  each 
other ;  and  it  supplies  precise  and  exact  numerical 
relations  between  the  magnitudes  which  enter  into 
these  phenomena. 

The  boldest  minds  have  an  instinctive  confidence 
in  it,  and  it  is  the  principle  which  has  most  stoutly 
resisted  that  assault  which  the  daring  of  a  few 
theorists  has  lately  directed  to  the  overthrow  of 
the  general  principles  of  physics.  At  every  new 
discovery,  the  first  thought  of  physicists  is  to  find 
out  how  it  accords  with  the  principle  of  the 
conservation  of  energy.  The  application  of  the 
principle,  moreover,  never  fails  to  give  valuable 
hints  on  the  new  phenomenon,  and  often  even 
suggests  a  complementary  discovery.  Up  till  now 
it  seems  never  to  have  received  a  check,  even  the 
extraordinary  properties  of  radium  not  seriously 
contradicting  it;  also  the  general  form  in  which  it 
is  enunciated  gives  it  such  a  suppleness  that  it  is 
no  doubt  very  difficult  to  overthrow. 

I  do  not  claim  to  set  forth  here  the  complete 
history  of  this  principle,  but  I  will  endeavour  to 
show  with  what  pains  it  was  born,  how  it  was  kept 
back  in  its  early  days  and  then  obstructed  in  its 
development  by  the  unfavourable  conditions  of  the 


PRINCIPLES  57 

surroundings  in  which  it  appeared.  It  first  of  all 
came,  in  fact,  to  oppose  itself  to  the  reigning 
theories;  but,  little  by  little,  it  acted  on  these 
theories,  and  they  were  modified  under  its  pressure ; 
then,  in  their  turn,  these  theories  reacted  on  it  and 
changed  its  primitive  form. 

It  had  to  be  made  less  wide  in  order  to  fit  into  the 
classic  frame,  and  was  absorbed  by  mechanics ;  and 
if  it  thus  became  less  general,  it  gained  in  precision 
what  it  lost  in  extent.  When  once  definitely  admitted 
and  classed,  as  it  were,  in  the  official  domain  of 
science,  it  endeavoured  to  burst  its  bonds  and  return 
to  a  more  independent  and  larger  life.  The  history 
of  this  principle  is  similar  to  that  of  all  evolutions. 

It  is  well  known  that  the  conservation  of  energy 
was,  at  first,  regarded  from  the  point  of  view  of  the 
reciprocal  transformations  between  heat  and  work, 
and  that  the  principle  received  its  first  clear 
enunciation  in  the  particular  case  of  the  principle 
of  equivalence.  It  is,  therefore,  rightly  considered 
that  the  scholars  who  were  the  first  to  doubt  the 
material  nature  of  caloric  were  the  precursors  of 
K.  Mayer ;  their  ideas,  however,  were  the  same  as 
those  of  the  celebrated  German  doctor,  for  they 
sought  especially  to  demonstrate  that  heat  was  a 
mode  of  motion. 

Without  going  back  to  early  and  isolated 
attempts  like  those  of  Daniel  Bernoulli,  who,  in 
his  hydrodynamics,  propounded  the  basis  of  the 


5  8     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

kinetic  theory  of  gases,  or  the  researches  of  Boyle 
on  friction,  we  may  recall,  to  show  how  it  was  pro- 
pounded in  former  times,  a  rather  forgotten  page  of 
the  Mtmoire  sur  la  Chaleur,  published  in  1780  by 
Lavoisier  and  Laplace :  "  Other  physicists,"  they 
wrote,  after  setting  out  the  theory  of  caloric,  "  think 
that  heat  is  nothing  but  the  result  of  the  insensible 
vibrations  of  matter.  ...  In  the  system  we  are  now 
examining,  heat  is  the  vis  viva  resulting  from  the 
insensible  movements  of  the  molecules  of  a  body ;  it 
is  the  sum  of  the  products  of  the  mass  of  each 
molecule  by  the  square  of  its  velocity.  .  .  .  We 
shall  not  decide  between  the  two  preceding  hypo- 
theses; several  phenomena  seem  to  support  the 
last  mentioned — for  instance,  that  of  the  heat 
produced  by  the  friction  of  two  solid  bodies.  But 
there  are  others  which  are  more  simply  explained 
by  the  first,  and  perhaps  they  both  operate  at 
once."  Most  of  the  physicists  of  that  period,  how- 
ever, did  not  share  the  prudent  doubts  of  Lavoisier 
and  Laplace.  They  admitted,  without  hesitation,  the 
first  hypothesis ;  and,  four  years  after  the  appearance 
of  the  Me'moire  sur  la  Chaleur,  Sigaud  de  Lafond,  a 
professor  of  physics  of  great  reputation,  wrote :  "  Pure 
Fire,  free  from  all  state  of  combination,  seems  to  be 
an  assembly  of  particles  of  a  simple,  homogeneous, 
and  absolutely  unalterable  matter,  and  all  the 
properties  of  this  element  indicate  that  these 
particles  are  infinitely  small  and  free,  that  they  have 


PRINCIPLES  59 

no  sensible  cohesion,  and  that  they  are  moved  in 
every  possible  direction  by  a  continual  and  rapid 
motion  which  is  essential  to  them.  .  .  .  The  extreme 
tenacity  and  the  surprising  mobility  of  its  molecules 
are  manifestly  shown  by  the  ease  with  which  it 
penetrates  into  the  most  compact  bodies  and  by 
its  tendency  to  put  itself  in  equilibrium  throughout 
all  bodies  near  to  it." 

It  must  be  acknowledged,  however,  that  the  idea 
of  Lavoisier  and  Laplace  was  rather  vague  and  even 
inexact  on  one  important  point.  They  admitted  it 
to  be  evident  that  "  all  variations  of  heat,  whether 
real  or  apparent,  undergone  by  a  bodily  system 
when  changing  its  state,  are  produced  in  inverse 
order  when  the  system  passes  back  to  its  original 
state."  This  phrase  is  the  very  denial  of  equivalence 
where  these  changes  of  state  are  accompanied  by 
external  work. 

Laplace,  moreover,  himself  became  later  a  very 
convinced  partisan  of  the  hypothesis  of  the  material 
nature  of  caloric,  and  his  immense  authority,  so 
fortunate  in  other  respects  for  the  development  of 
science,  was  certainly  in  this  case  the  cause  of  the 
retardation  of  progress. 

The  names  of  Young,  Eumford,  Davy,  are  often 
quoted  among  those  physicists  who,  at  the  com- 
mencement of  the  nineteenth  century,  caught  sight 
of  the  new  truths  as  to  the  nature  of  heat.  To  these 
names  is  very  properly  added  that  of  Sadi  Carnot. 


60     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

A  note  found  among  his  papers  unquestionably 
proves  that,  before  1830,  ideas  had  occurred  to  him 
from  -which  it  resulted  that  in  producing  work  an 
equivalent  amount  of  heat  was  destroyed.  But  the 
year  1842  is  particularly  memorable  in  the  history 
of  science  as  the  year  in  which  Jules  Eobert 
Mayer  succeeded,  by  an  entirely  personal  effort,  in 
really  enunciating  the  principle  of  the  conserva- 
tion of  energy.  Chemists  recall  with  just  pride  that 
the  Eemarques  sur  les  forces  de  la  nature  animfo, 
contemptuously  rejected  by  all  the  journals  of 
physics,  were  received  and  published  in  the  Anndlen 
of  Liebig.  We  ought  never  to  forget  this  example, 
which  shows  with  what  difficulty  a  new  idea  contrary 
to  the  classic  theories  of  the  period  succeeds  in 
coming  to  the  front ;  but  extenuating  circumstances 
may  be  urged  on  behalf  of  the  physicists. 

Kobert  Mayer  had  a  rather  insufficient  mathe- 
matical education,  and  his  Memoirs,  the  Memarques,  as 
well  as  the  ulterior  publications,  Mtmoire  sur  le  mouve- 
ment  organique  et  la  nutrition  and  the  MaUriaux 
pour  la  dynamique  du  del,  contain,  side  by  side 
with  very  profound  ideas,  evident  errors  in  mechanics. 
Thus  it  often  happens  that  discoveries  put  forward 
in  a  somewhat  vague  manner  by  adventurous 
minds  not  overburdened  by  the  heavy  baggage  of 
scientific  erudition,  who  audaciously  press  forward  in 
advance  of  their  time,  fall  into  quite  intelligible 
oblivion  until  rediscovered,  clarified,  and  put  into 


PKINCIPLES  61 

shape  by  slower  but  surer  seekers.  This  was  the 
case  with  the  ideas  of  Mayer.  They  were  not 
understood  at  first  sight,  not  only  on  account  of 
their  originality,  but  also  because  they  were  couched 
in  incorrect  language. 

Mayer  was,  however,  endowed  with  a  singular 
strength  of  thought;  he  expressed  in  a  rather 
confused  manner  a  principle  which,  for  him,  had  a 
generality  greater  than  mechanics  itself,  and  so  his 
discovery  was  in  advance  not  only  of  his  own  time 
but  of  half  the  century.  He  may  justly  be  con- 
sidered the  founder  of  modern  energetics. 

Freed  from  the  obscurities  which  prevented  its 
being  clearly  perceived,  his  idea  stands  out  to-day 
in  all  its  imposing  simplicity.  Yet  it  must  be 
acknowledged  that  if  it  was  somewhat  denaturalised 
by  those  who  endeavoured  to  adapt  it  to  the 
theories  of  mechanics,  and  if  it  at  first  lost  its 
sublime  stamp  of  generality,  it  thus  became  firmly 
fixed  and  consolidated  on  a  more  stable  basis. 

The  efforts  of  Helmholtz,  Clausius,  and  Lord 
Kelvin  to  introduce  the  principle  of  the  conserva- 
tion of  energy  into  mechanics,  were  far  from  useless. 
These  illustrious  physicists  succeeded  in  giving  a 
more  precise  form  to  its  numerous  applications ; 
and  their  attempts  thus  contributed,  by  reaction, 
to  give  a  fresh  impulse  to  mechanics,  and  allowed  it 
to  be  linked  to  a  more  general  order  of  facts.  If 
energetics  has  not  been  able  to  be  included  in 


62     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

mechanics,  it  seems  indeed  that  the  attempt  to 
include  mechanics  in  energetics  was  not  in  vain. 

In  the  middle  of  the  last  century,  the  explanation 
of  all  natural  phenomena  seemed  more  and  more 
referable  to  the  case  of  central  forces.  Everywhere 
it  was  thought  that  reciprocal  actions  between 
material  points  could  be  perceived,  these  points 
being  attracted  or  repelled  by  each  other  with  an 
intensity  depending  only  on  their  distance  or  their 
mass.  If,  to  a  system  thus  composed,  the  laws  of  the 
classical  mechanics  are  applied,  it  is  shown  that  half 
the  sum  of  the  product  of  the  masses  by  the  square 
of  the  velocities,  to  which  is  added  the  work  which 
might  be  accomplished  by  the  forces  to  which  the 
system  would  be  subject  if  it  returned  from  its 
actual  to  its  initial  position,  is  a  sum  constant  in 
quantity. 

This  sum,  which  is  the  mechanical  energy  of  the 
system,  is  therefore  an  invariable  quantity  in  all 
the  states  to  which  it  may  be  brought  by  the  inter- 
action of  its  various  parts,  and  the  word  energy 
well  expresses  a  capital  property  of  this  quantity. 
For  if  two  systems  are  connected  in  such  a  way 
that  any  change  produced  in  the  one  necessarily 
brings  about  a  change  in  the  other,  there  can  be 
no  variation  in  the  characteristic  quantity  of  the 
second  except  so  far  as  the  characteristic  quantity 
of  the  first  itself  varies — on  condition,  of  course, 
that  the  connexions  are  made  in  such  a  manner 


PRINCIPLES  63 

as  to  introduce  no  new  force.  It  will  thus 
be  seen  that  this  quantity  well  expresses  the 
capacity  possessed  by  a  system  for  modifying  the 
state  of  a  neighbouring  system  to  which  we  may 
suppose  it  connected. 

Now  this  theorem  of  pure  mechanics  was  found 
wanting  every  time  friction  took  place — that  is  to 
say,  in  all  really  observable  cases.  The  more  per- 
ceptible the  friction,  the  more  considerable  the 
difference ;  but,  in  addition,  a  new  phenomenon  always 
appeared  and  heat  was  produced.  By  experiments 
which  are  now  classic,  it  became  established  that 
the  quantity  of  heat  thus  created  independently  of 
the  nature  of  the  bodies  is  always  (provided  no  other 
phenomena  intervene)  proportional  to  the  energy 
which  has  disappeared.  Reciprocally,  also,  heat 
may  disappear,  and  we  always  find  a  constant 
relation  between  the  quantities  of  heat  and  work 
which  mutually  replace  each  other. 

It  is  quite  clear  that  such  experiments  do  not 
prove  that  heat  is  work.  We  might  just  as  well  say 
that  work  is  heat.  It  is  making  a  gratuitous  hypo- 
thesis to  admit  this  reduction  of  heat  to  mechanism ; 
but  this  hypothesis  was  so  seductive,  and  so  much 
in  conformity  with  the  desire  of  nearly  all  physicists 
to  arrive  at  some  sort  of  unity  in  nature,  that  they 
made  it  with  eagerness  and  became  unreservedly 
convinced  that  heat  was  an  active  internal  force. 
Their  error  was  not  in  admitting  this  hypothesis  ; 


64     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

it  was  a  legitimate  one  since  it  has  proved  very 
fruitful.  But  some  of  them  committed  the  fault  of 
forgetting  that  it  was  an  hypothesis,  and  considered 
it  a  demonstrated  truth.  Moreover,  they  were  thus 
brought  to  see  in  phenomena  nothing  but  these  two 
particular  forms  of  energy  which  in  their  minds 
were  easily  identified  with  each  other. 

From  the  outset,  however,  it  became  manifest 
that  the  principle  is  applicable  to  cases  where 
heat  plays  only  a  parasitical  part.  There  were 
thus  discovered,  by  translating  the  principle  of 
equivalence,  numerical  relations  between  the  magni- 
tudes of  electricity,  for  instance,  and  the  magnitudes 
of  mechanics.  Heat  was  a  sort  of  variable  inter- 
mediary convenient  for  calculation,  but  introduced 
in  a  roundabout  way  and  destined  to  disappear  in 
the  final  result. 

Verdet,  who,  in  lectures  which  have  rightly 
remained  celebrated,  defined  with  remarkable  clear- 
ness the  new  theories,  said,  in  1862:  "Electrical 
phenomena  are  always  accompanied  by  calorific 
manifestations,  of  which  the  study  belongs  to  the 
mechanical  theory  of  heat.  This  study,  moreover, 
will  not  only  have  the  effect  of  making  known 
to  us  interesting  facts  in  electricity,  but  will 
throw  some  light  on  the  phenomena  of  electricity 
themselves." 

The  eminent  professor  was  thus  expressing  the 
general  opinion  of  his  contemporaries,  but  he  certainly 


PRINCIPLES  65 

seemed  to  have  felt  in  advance  that  the  new 
theory  was  about  to  penetrate  more  deeply  into 
the  inmost  nature  of  things.  Three  years  previ- 
ously, Eankine  also  had  put  forth  some  very  remark- 
able ideas  the  full  meaning  of  which  was  not  at 
first  well  understood.  He  it  was  who  compre- 
hended the  utility  of  employing  a  more  inclusive 
term,  and  invented  the  phrase  energetics.  He  also 
endeavoured  to  create  a  new  doctrine  of  which 
rational  mechanics  should  be  only  a  particular 
case ;  and  he  showed  that  it  was  possible  to  aban- 
don the  ideas  of  atoms  and  central  forces,  and  to 
construct  a  more  general  system  by  substituting  for 
the  ordinary  consideration  of  forces  that  of  the 
energy  which  exists  in  all  bodies,  partly  in  an  actual, 
partly  in  a  potential  state. 

By  giving  more  precision  to  the  conceptions  of 
Kankine,  the  physicists  of  the  end  of  the  nineteenth 
century  were  brought  to  consider  that  in  all  physical 
phenomena  there  occur  apparitions  and  disappear- 
ances which  are  balanced  by  various  energies.  It 
is  natural,  however,  to  suppose  that  these  equivalent 
apparitions  and  disappearances  correspond  to  trans- 
formations and  not  to  simultaneous  creations  and 
destructions.  We  thus  represent  energy  to  ourselves 
as  taking  different  forms — mechanical,  electrical, 
calorific,  and  chemical — capable  of  changing  one  into 
the  other,  but  in  such  a  way  that  the  quantitative 
value  always  remains  the  same.  In  like  manner  a 

5 


66     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

bank  draft  may  be  represented  by  notes,  gold,  silver, 
or  bullion.  The  earliest  known  form  of  energy,  i.e. 
work,  will  serve  as  the  standard  as  gold  serves  as  the 
monetary  standard,  and  energy  in  all  its  forms  will 
be  estimated  by  the  corresponding  work.  In  each 
particular  case  we  can  strictly  define  and  measure, 
by  the  correct  application  of  the  principle  of  -the 
conservation  of  energy,  the  quantity  of  energy 
evolved  under  a  given  form. 

We  can  thus  arrange  a  machine  comprising  a 
body  capable  of  evolving  this  energy ;  then  we  can 
force  all  the  organs  of  this  machine  to  complete 
an  entirely  closed  cycle,  with  the  exception  of  the 
body  itself,  which,  however,  has  to  return  to  such  a 
state  that  all  the  variables  from  which  this  state 
depends  resume  their  initial  values  except  the 
particular  variable  to  which  the  evolution  of  the 
energy  under  consideration  is  linked.  The  difference 
between  the  work  thus  accomplished  and  that  which 
would  have  been  obtained  if  this  variable  also  had 
returned  to  its  original  value,  is  the  measure  of  the 
energy  evolved. 

In  the  same  way  that,  in  the  minds  of  mechani- 
cians, all  forces  of  whatever  origin,  which  are  capable 
of  compounding  with  each  other  and  of  balancing 
each  other,  belong  to  the  same  category  of  beings, 
so  for  many  physicists  energy  is  a  sort  of  entity 
which  we  find  under  various  aspects.  There  thus 
exists  for  them  a  world,  which  comes  in  some  way 


PRINCIPLES  67 

to  superpose  itself  upon  the  world  of  matter — that 
is  to  say,  the  world  of  energy,  dominated  in  its  turn 
by  a  fundamental  law  similar  to  that  of  Lavoisier.1 
This  conception,  as  we  have  already  seen,  passes 
the  limit  of  experience ;  but  others  go  further  still. 
Absorbed  in  the  contemplation  of  this  new  world, 
they  succeed  in  persuading  themselves  that  the 
old  world  of  matter  has  no  real  existence  and  that 
energy  is  sufficient  by  itself  to  give  us  a  complete 
comprehension  of  the  Universe  and  of  all  the 
phenomena  produced  in  it.  They  point  out  that  all 
our  sensations  correspond  to  changes  of  energy,  and 
that  everything  apparent  to  our  senses  is,  in  truth, 
energy.  The  famous  experiment  of  the  blows  with 
a  stick  by  which  it  was  demonstrated  to  a  sceptical 
philosopher  that  an  outer  world  existed,  only  proves, 
in  reality,  the  existence  of  energy,  and  not  that 
of  matter.  The  stick  in  itself  is  inoffensive,  as 
Professor  Ostwald  remarks,  and  it  is  its  vis  viva,  its 
kinetic  energy,  which  is  painful  .to  us ;  while  if  we 
possessed  a  speed  equal  to  its  own,  moving  in  the 
same  direction,  it  would  no  longer  exist  so  far  as  our 
sense  of  touch  is  concerned. 

On  this  hypothesis,  matter  would  only  be  the 
capacity  for  kinetic  energy,  its  pretended  impene- 
trability energy  of  volume,  and  its  weight  energy  of 
position  in  the  particular  form  which  presents  itself 
in  universal  gravitation;  nay,  space  itself  would 
1  "  Nothing  is  created  ;  nothing  is  lost." — ED. 


68    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

only  be  known  to  us  by  the  expenditure  of  energy 
necessary  to  penetrate  it.  Thus  in  all  physical 
phenomena  we  should  only  have  to  regard  the 
quantities  of  energy  brought  into  play,  and  all  the 
equations  which  link  the  phenomena  to  one  another 
would  have  no  meaning  but  when  they  apply  to 
exchanges  of  energy.  For  energy  alone  can  be 

common  to  all  phenomena. 

i 

This  extreme  manner  of  regarding  things  is 
seductive  by  its  originality,  but  appears  somewhat 
insufficient  if,  after  enunciating  generalities,  we  look 
more  closely  into  the  question.  From  the  philo- 
sophical point  of  view  it  may,  moreover,  seem  diffi- 
cult not  to  conclude,  from  the  qualities  which  reveal, 
if  you  will,  the  varied  forms  of  energy,  that  there 
exists  a  substance  possessing  these  qualities.  This 
energy,  which  resides  in  one  region,  and  which 
transports  itself  from  one  spot  to  another,  forcibly 
brings  to  mind,  whatever  view  we  may  take  of  it, 
the  idea  of  matter. 

Helmholtz  endeavoured  to  construct  a  mechanics 
based  on  the  idea  of  energy  and  its  conservation, 
but  he  had  to  invoke  a  second  law,  the  principle  of 
least  action.  If  he  thus  succeeded  in  dispensing 
with  the  hypothesis  of  atoms,  and  in  showing  that 
the  new  mechanics  gave  us  to  understand  the  im- 
possibility of  certain  movements  which,  according 
to  the  old,  ought  to  have  been  but  never  were  ex- 
perimentally produced,  he  was  only  able  to  do  so 


PRINCIPLES  69 

because  the  principle  of  least  action  necessary  for 
his  theory  became  evident  in  the  case  of  those 
irreversible  phenomena  which  alone  really  exist 
in  Nature.  The  energetists  have  thus  not  succeeded 
in  forming  a  thoroughly  sound  system,  but  their 
efforts  have  at  all  events  been  partly  successful. 
Most  physicists  are  of  their  opinion,  that  kinetic 
energy  is  only  a  particular  variety  of  energy  to 
which  we  have  no  right  to  wish  to  connect  all 
its  other  forms. 

If  these  forms  showed  themselves  to  be  innumer- 
able throughout  the  Universe,  the  principle  of  the 
conservation  of  energy  would,  in  fact,  lose  a  great 
part  of  its  importance.  Every  time  that  a  certain 
quantity  of  energy  seemed  to  appear  or  disappear, 
it  would  always  be  permissible  to  suppose  that  an 
equivalent  quantity  had  appeared  or  disappeared 
somewhere  else  under  a  new  form  ;  and  thus  the 
principle  would  in  a  way  vanish.  But  the  known 
forms  of  energy  are  fairly  restricted  in  number,  and 
the  necessity  of  recognising  new  ones  seldom  makes 
itself  felt.  We  shall  see,  however,  that  to  explain,  for 
instance,  the  paradoxical  properties  of  radium  and 
to  re-establish  concord  between  these  properties  and 
the  principle  of  the  conservation  of  energy,  certain 
physicists  have  recourse  to  the  hypothesis  that 
radium  borrows  an  unknown  energy  from  the 
medium  in  which  it  is  plunged.  This  hypothesis, 
however,  is  in  no  way  necessary  ;  and  in  a  few  other 


70    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

rare  cases  in  which  similar  hypotheses  have  had  to 
be  set  up,  experiment  has  always  in  the  long  run 
enabled  us  to  discover  some  phenomenon  which  had 
escaped  the  first  observers  and  which  corresponds 
exactly  to  the  variation  of  energy  first  made 
evident. 

One  difficulty,  however,  arises  from  the  fact  that 
the  principle  ought  only  to  be  applied  to  an  isolated 
system.  Whether  we  imagine  actions  at  a  distance 
or  believe  in  intermediate  media,  we  must  always 
recognise  that  there  exist  no  bodies  in  the  world 
incapable  of  acting  on  each  other,  and  we  can  never 
affirm  that  some  modification  in  the  energy  of  a 
given  place  may  not  have  its  echo  in  some  unknown 
spot  afar  off.  This  difficulty  may  sometimes,  render 
the  value  of  the  principle  rather  illusory. 

Similarly,  it  behoves  us  not  to  receive  without  a 
certain  distrust  the  extension  by  certain  philosophers 
to  the  whole  Universe,  of  a  property  demonstrated 
for  those  restricted  systems  which  observation  can 
alone  reach.  We  know  nothing  of  the  Universe  as 
a  whole,  and  every  generalization  of  this  kind  out- 
runs in  a  singular  fashion  the  limit  of  experiment. 

Even  reduced  to  the  most  modest  proportions, 
the  principle  of  the  conservation  of  energy  retains, 
nevertheless,  a  paramount  importance ;  and  it  still 
preserves,  if  you  will,  a  high  philosophical  value. 
M.  J.  Perrin  justly  points  out  that  it  gives  us  a 
form  under  which  we  are  experimentally  able  to 


PRINCIPLES  71 

grasp  causality,  and  that  it  teaches  us  that  a  result 
has  to  be  purchased  at  the  cost  of  a  determined 
effort. 

We  can,  in  fact,  with  M.  Perrin  and  M.  Langevin, 
represent  this  in  a  way  which  puts  this  character- 
istic in  evidence  by  enunciating  it  as  follows :  "  If 
at  the  cost  of  a  change  C  we  can  obtain  a  change  K, 
there  will  never  be  acquired  at  the  same  cost,  what- 
ever the  mechanism  employed,  first  the  change  K 
and  in  addition  some  other  change,  unless  this  latter 
be  one  that  is  otherwise  known  to  cost  nothing  to 
produce  or  to  destroy."  If,  for  instance,  the  fall 
of  a  weight  can  be  accompanied,  without  anything 
else  being  produced,  by  another  transformation — the 
melting  of  a  certain  mass  of  ice,  for  example — it  will 
be  impossible,  no  matter  how  you  set  about  it  or 
whatever  the  mechanism  used,  to  associate  this  same 
transformation  with  the  melting  of  another  weight 
of  ice. 

We  can  thus,  in  the  transformation  in  question, 
obtain  an  appropriate  number  which  will  sum  up 
that  which  may  be  expected  from  the  external  effect, 
and  can  give,  so  to  speak,  the  price  at  which  this 
transformation  is  bought,  measure  its  invariable 
value  by  a  common  measure  (for  instance,  the 
melting  of  the  ice),  and,  without  any  ambiguity, 
define  the  energy  lost  during  the  transformation  as 
proportional  to  the  mass  of  ice  which  can  be 
associated  with  it.  This  measure  is,  moreover, 


72    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

independent  of  the  particular  phenomenon  taken  as 
the  common  measure. 

§  3.  THE  PRINCIPLE  OF  CARNOT  AND  CLAUSIUS 

The  principle  of  Carnot,  of  a  nature  analogous  to 
the  principle  of  the  conservation  of  energy,  has  also  a 
similar  origin.  It  was  first  enunciated,  like  the  last 
named,  although  prior  to  it  in  time,  in  consequence 
of  considerations  which  deal  only  with  heat  and 
mechanical  work.  Like  it,  too,  it  has  evolved, 
grown,  and  invaded  the  entire  domain  of  physics. 
It  may  be  interesting  to  examine  rapidly  the  various 
phases  of  this  evolution.  The  origin  of  the  principle 
of  Carnot  is  clearly  determined,  and  it  is  very  rare 
to  be  able  to  go  back  thus  certainly  to  the  source 
of  a  discovery.  Sadi  Carnot  had,  truth  to  say,  no 
precursor.  In  his  time  heat  engines  were  not  yet 
very  common,  and  no  one  had  reflected  much  on 
their  theory.  He  was  doubtless  the  first  to  pro- 
pound to  himself  certain  questions,  and  certainly  the 
first  to  solve  them. 

It  is  known  how,  in  1824,  in  his  Reflexions  sur  la 
puissance  motrice  du  feu,  he  endeavoured  to  prove 
that  "the  motive  power  of  heat  is  independent  of 
the  agents  brought  into  play  for  its  realization,"  and 
that  "  its  quantity  is  fixed  solely  by  the  temperature 
of  the  bodies  between  which,  in  the  last  resort,  the 
transport  of  caloric  is  effected " — at  least  in  all 
engines  in  which  "  the  method  of  developing  the 


PKINCIPLES  73 

motive  power  attains  the  perfection  of  which  it  is 
capable";  and  this  is,  almost  textually,  one  of  the 
enunciations  of  the  principle  at  the  present  day. 
Carnot  perceived  very  clearly  the  great  fact  that,  to 
produce  work  by  heat,  it  is  necessary  to  have  at 
one's  disposal  a  fall  of  temperature.  On  this  point 
he  expresses  himself  with  perfect  clearness :  "  The 
motive  power  of  a  fall  of  water  depends  on  its  height 
and  on  the  quantity  of  liquid  ;  the  motive  power  of 
heat  depends  also  on  the  quantity  of  caloric  em- 
ployed, and  on  what  might  be  called — in  fact,  \\hat 
we  shall  call — the  height  of  fall,  that  is  to  say,  the 
difference  in  temperature  of  the  bodies  between  which 
the  exchange  of  caloric  takes  place. " 

Starting  with  this  idea,  he  endeavours  to  demon- 
strate, by  associating  two  engines  capable  of  working 
in  a  reversible  cycle,  that  the  principle  is  founded  on 
the  impossibility  of  perpetual  motion. 

His  memoir,  now  celebrated,  did  not  produce  any 
great  sensation,  and  it  had  almost  fallen  into  deep 
oblivion,  which,  in  consequence  of  the  discovery  of 
the  principle  of  equivalence,  might  have  seemed 
perfectly  justified.  Written,  in  fact,  on  the  hypo- 
thesis of  the  indestructibility  of  caloric,  it  was  to 
be  expected  that  this  memoir  should  be  condemned 
in  the  name  of  the  new  doctrine,  that  is,  of  the 
principle  recently  brought  to  light. 

It  was  really  making  a  new  discovery  to  establish 
that  Carnot's  fundamental  idea  survived  the  destruc- 


74    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

tion  of  the  hypothesis  on  the  nature  of  heat,  on 
which  he  seemed  to  rely.  As  he  no  doubt  himself 
perceived,  his  idea  was  quite  independent  of  this 
hypothesis,  since,  as  we  have  seen,  he  was  led  to 
surmise  that  heat  could  disappear;  but  his  demon- 
strations needed  to  be  recast  and,  in  some  points, 
modified. 

It  is  to  Clausius  that  was  reserved  the  credit  of 
rediscovering  the  principle,  and  of  enunciating  it 
in  language  conformable  to  the  new  doctrines,  while 
giving  it  a  much  greater  generality.  The  postulate 
arrived  at  by  experimental  induction,  and  which 
must  be  admitted  without  demonstration,  is,  accord- 
ing to  Clausius,  that  in  a  series  of  transforma- 
tions in  which  the  final  is  identical  with  the  initial 
stage,  it  is  impossible  for  heat  to  pass  from  a  colder 
to  a  warmer  body  unless  some  other  accessory 
phenomenon  occurs  at  the  same  time. 

Still  more  correctly,  perhaps,  an  enunciation  can 
be  given  of  the  postulate  which,  in  the  main,  is 
analogous,  by  saying:  A  heat  motor,  which  after 
a  series  of  transformations  returns  to  its  initial 
state,  can  only  furnish  work  if  there  exist  at  least 
two  sources  of  heat,  and  if  a  certain  quantity  of 
heat  is  given  to  one  of  the  sources,  which  can  never 
be  the  hotter  of  the  two.  By  the  expression  "  source 
of  heat,"  we  mean  a  body  exterior  to  the  system  and 
capable  of  furnishing  or  withdrawing  heat  from  it. 

Starting   with   this   principle,  we   arrive,  as  does 


PRINCIPLES  75 

Clausius,  at  the  demonstration  that  the  output  of 
a  reversible  machine  working  between  two  given 
temperatures  is  greater  than  that  of  any  non-re- 
versible engine,  and  that  it  is  the  same  for  all 
reversible  machines  working  between  these  two 
temperatures. 

This  is  the  very  proposition  of  Car  not ;  but  the 
proposition  thus  stated,  while  very  useful  for  the 
theory  of  engines,  does  not  yet  present  any  very 
general  interest.  Clausius,  however,  drew  from 
it  much  more  important  consequences.  First,  he 
showed  that  the  principle  conduces  to  the  definition 
of  an  absolute  scale  of  temperature ;  and  then  he 
was  brought  face  to  face  with  a  new  notion  which 
allows  a  strong  light  to  be  thrown  on  the  questions 
of  physical  equilibrium.  I  refer  to  entropy. 

It  is  still  rather  difficult  to  strip  entirely  this 
very  important  notion  of  all  analytical  adornment. 
Many  physicists  hesitate  to  utilize  it,  and  even 
look  upon  it  with  some  distrust,  because  they  see 
in  it  a  purely  mathematical  function  without  any 
definite  physical  meaning.  Perhaps  they  are  here 
unduly  severe,  since  they  often  admit  too  easily 
the  objective  existence  of  quantities  which  they 
cannot  define.  Thus,  for  instance,"  it  is  usual  almost 
every  day  to  speak  of  the  heat  possessed  by  a 
body.  Yet  no  body  in  reality  possesses  a  definite 
quantity  of  heat  even  relatively  to  any  initial  state  ;, 
since  starting  from  this  point  of  departure,  the 


76    THE  NEW  PHYSICS  AND  ITS   EVOLUTION 

quantities  of  heat  it  may  have  gained  or  lost  vary 
with  the  road  taken  and  even  with  the  means 
employed  to  follow  it.  These  expressions  of  heat 
gained  or  lost  are,  moreover,  themselves  evidently 
incorrect,  for  heat  can  no  longer  be  considered  as  a 
sort  of  fluid  passing  from  one  body  to  another. 

The  real  reason  which  makes  entropy  somewhat 
mysterious  is  that  this  magnitude  does  not  fall 
directly  under  the  ken  of  any  of  our  senses ;  but  it 
possesses  the  true  characteristic  of  a  concrete  physical 
magnitude,  since  it  is,  in  principle  at  least,  measur- 
able. Various  authors  of  thermodynamical  re- 
searches, amongst  whom  M.  Mouret  should  be  parti- 
cularly mentioned,  have  endeavoured  to  place  this 
characteristic  in  evidence. 

Consider  an  isothermal  transformation.  Instead 
of  leaving  the  heat  abandoned  by  the  body  sub- 
jected to  the  transformation — water  condensing  in 
a  state  of  saturated  vapour,  for  instance — to  pass 
directly  into  an  ice  calorimeter,  we  can  transmit 
this  heat  to  the  calorimeter  by  the  intermediary 
of  a  reversible  Carnot  engine.  The  engine  having 
absorbed  this  quantity  of  heat,  will  only  give  back 
to  the  ice  a  lesser  quantity  of  heat ;  and  the  weight 
of  the  melted  ice,  inferior  to  that  which  might  have 
been  directly  given  back,  will  serve  as  a  measure  of 
the  isothermal  transformation  thus  effected.  It  can 
be  easily  shown  that  this  measure  is  independent  of 
the  apparatus  used.  It  consequently  becomes  a 


PRINCIPLES  77 

numerical  element  characteristic  of  the  body  con- 
sidered, and  is  called  its  entropy.  Entropy,  thus 
defined,  is  a  variable  which,  like  pressure  or  volume, 
might  serve  concurrently  with  another  variable,  such 
as  pressure  or  volume,  to  define  the  state  of  a  body. 

It  must  be  perfectly  understood  that  this  variable 
can  change  in  an  independent  manner,  and  that  it  is, 
for  instance,  distinct  from  the  change  of  temperature. 
It  is  also  distinct  from  the  change  which  consists  in 
losses  or  gains  of  heat.  In  chemical  reactions,  for 
example,  the  entropy  increases  without  the  substances 
borrowing  any  heat.  When  a  perfect  gas  dilates  in 
a  vacuum  its  entropy  increases,  and  yet  the  tempera- 
ture does  not  change,  and  the  gas  has  neither  been 
able  to  give  nor  receive  heat.  We  thus  come  to 
conceive  that  a  physical  phenomenon  cannot  be  con- 
sidered known  to  us  if  the  variation  of  entropy  is 
not  given,  as  are  the  variations  of  temperature  and 
of  pressure  or  the  exchanges  of  heat.  The  change 
of  entropy  is,  properly  speaking,  the  most  character- 
istic fact  of  a  thermal  change. 

It  is  important,  however,  to  remark  that  if  we 
can  thus  easily  define  and  measure  the  difference  of 
entropy  between  two  states  of  the  same  body,  the 
value  found  depends  on  the  state  arbitrarily  chosen 
as  the  zero  point  of  entropy ;  but  this  is  not  a  very 
serious  difficulty,  and  is  analogous  to  that  which 
occurs  in  the  evaluation  of  other  physical  magnitudes 
— temperature,  potential,  etc. 


78    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

A  graver  difficulty  proceeds  from  its  not  being 
possible  to  define  a  difference,  or  an  equality,  of 
entropy  between  two  bodies  chemically  different. 
We  are  unable,  in  fact,  to  pass  by  any  means, 
reversible  or  not,  from  one  to  the  other,  so  long  as 
the  transmutation  of  matter  is  regarded  as  impossible ; 
but  it  is  well  understood  that  it  is  nevertheless  pos- 
sible to  compare  the  variations  of  entropy  to  which 
these  two  bodies  are  both  of  them  individually  subject. 

Neither  must  we  conceal  from  ourselves  that  the 
definition  supposes,  for  a  given  body,  the  possi- 
bility of  passing  from  one  state  to  another  by  a 
reversible  transformation.  Reversibility  is  an  ideal 
and  extreme  case  which  cannot  be  realized,  but 
which  can  be  approximately  attained  in  many  cir- 
cumstances. So  with  gases  and  with  perfectly  elastic 
bodies,  we  effect  sensibly  reversible  transformations, 
and  changes  of  physical  state  are  practically  rever- 
sible. The  discoveries  of  Sainte-Claire  Deville  have 
brought  many  chemical  phenomena  into  a  similar 
category,  and  reactions  such  as  solution,  which  used 
to  be  formerly  the  type  of  an  irreversible  pheno- 
menon, may  now  often  be  effected  by  sensibly  re- 
versible means.  Be  that  as  it  may,  when  once  the 
definition  is  admitted,  we  arrive,  by  taking  as  a  basis 
the  principles  set  forth  at  the  inception,  at  the 
demonstration  of  the  celebrated  theorem  of  Clausius : 
The  entropy  of  a  thermally  isolated  system  continues 
to  increase  incessantly. 


PRINCIPLES  79 

It  is  very  evident  that  the  theorem  can  only  be 
worth  applying  in  cases  where  the  entropy  can  be 
exactly  defined  ;  but,  even  when  thus  limited,  the 
field  still  remains  vast,  and  the  harvest  which  we 
can  there  reap  is  very  abundant. 

Entropy  appears,  then,  as  a  magnitude  measuring 
in  a  certain  way  the  evolution  of  a  system,  or,  at 
least,  as  giving  the  direction  of  this  evolution.  This 
very  important  consequence  certainly  did  not  escape 
Clausius,  since  the  very  name  of  entropy,  which  he 
chose  to  designate  this  magnitude,  itself  signifies 
evolution.  We  have  succeeded  in  defining  this 
entropy  by  demonstrating,  as  has  been  said,  a  cer- 
tain number  of  propositions  which  spring  from  the 
postulate  of  Clausius;  it  is,  therefore,  natural  to 
suppose  that  this  postulate  itself  contains  in  potentid 
the  very  idea  of  a  necessary  evolution  of  physical 
systems.  But  as  it  was  first  enunciated,  it  contains 
it  in  a  deeply  hidden  way. 

No  doubt  we  should  make  the  principle  of  Carnot 
appear  in  an  interesting  light  by  endeavouring  to 
disengage  this  fundamental  idea,  and  by  placing  it, 
as  it  were,  in  large  letters.  Just  as,  in  elementary 
geometry,  we  can  replace  the  postulate  of  Euclid  by 
other  equivalent  propositions,  so  the  postulate  of 
thermodynamics  is  not  necessarily  fixed,  and  it  is 
instructive  to  try  to  give  it  the  most  general  and 
suggestive  character. 

MM.  Perrin  and  Langevin  have  made  a  success- 


So    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

ful  attempt  in  this  direction.  M.  Perrin  enunciates 
the  following  principle:  An  isolated  system  never 
passes  twice  through  the  same  state.  In  this  form, 
the  principle  affirms  that  there  exists  a  necessary 
order  in  the  succession  of  two  phenomena;  that 
evolution  takes  place  in  a  determined  direction. 
If  you  prefer  it,  it  may  be  thus  stated:  Of  two 
converse  transformations  unaccompanied  by  any  ex- 
ternal effect,  one  only  is  possible.  For  instance,  two 
gases  may  diffuse  themselves  one  in  the  other  in 
constant  volume,  but  they  could  not  conversely 
separate  themselves  spontaneously. 

Starting  from  the  principle  thus  put  forward, 
we  make  the  logical  deduction  that  one  cannot  hope 
to  construct  an  engine  which  should  work  for  an 
indefinite  time  by  heating  a  hot  source  and  by 
cooling  a  cold  one.  We  thus  come  again  into  the 
route  traced  by  Clausius,  and  from  this  point  we  may 
follow  it  strictly. 

Whatever  the  point  of  view  adopted,  whether  we 
regard  the  proposition  of  M.  Perrin  as  the  corollary 
of  another  experimental  postulate,  or  whether  we 
consider  it  as  a  truth  which  we  admit  a  priori  and 
verify  through  its  consequences,  we  are  led  to 
consider  that  in  its  entirety  the  principle  of  Carnot 
resolves  itself  into  the  idea  that  we  cannot  go  back 
along  the  course  of  life,  and  that  the  evolution  of 
a  system  must  follow  its  necessary  progress. 

Clausius  and  Lord  Kelvin  have  drawn  from  these 


PRINCIPLES  8 1 

considerations  certain  well-known  consequences  on 
the  evolution  of  the  Universe.  Noticing  that 
entropy  is  a  property  added  to  matter,  they 
admit  that  there  is  in  the  world  a  total  amount 
of  entropy ;  and  as  all  real  changes  which  are 
produced  in  any  system  correspond  to  an  increase 
of  entropy,  it  may  be  said  that  the  entropy 
of  the  world  is  continually  increasing.  Thus 
the  quantity  of  energy  existing  in  the  Universe 
remains  constant,  but  transforms  itself  little  by  little 
into  heat  uniformly  distributed  at  a  temperature 
everywhere  identical.  In  the  end,  therefore,  there 
will  be  neither  chemical  phenomena  nor  manifesta- 
tion of  life ;  the  world  will  still  exist,  but  without 
motion,  and,  so  to  speak,  dead. 

These  consequences  must  be  admitted  to  be  very 
doubtful;  we  cannot  in  any  certain  way  apply  to 
the  Universe,  which  is  not  a  finite  system,  a  pro- 
position demonstrated,  and  that  not  unreservedly, 
in  the  sharply  limited  case  of  a  finite  system. 
Herbert  Spencer,  moreover,  in  his  book  on  First 
Principles,  brings  out  with  much  force  the  idea  that, 
even  if  the  Universe  came  to  an  end,  nothing  would 
allow  us  to  conclude  that,  once  at  rest,  it  would 
remain  so  indefinitely.  We  may  recognise  that  the 
state  in  which  we  are  began  at  the  end  of  a  former 
evolutionary  period,  and  that  the  end  of  the 
existing  era  will  mark  the  beginning  of  a  new  one. 

Like  an  elastic  and  mobile  object  which,  thrown 


$2    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

into  the  air,  attains  by  degrees  the  summit  of  its 
course,  then  possesses  a  zero  velocity  and  is  for  a 
moment  in  equilibrium,  and  then  falls  on  touching  the 
ground  to  rebound,  so  the  world  should  be  subjected 
to  huge  oscillations  which  first  bring  it  to  a  maximum 
of  entropy  till  the  moment  when  there  should  be 
produced  a  slow  evolution  in  the  contrary  direction 
bringing  it  back  to  the  state  from  which  it  started. 
Thus,  in  the  infinity  of  time,  the  life  of  the  Universe 
proceeds  without  real  stop. 

This  conception  is,  moreover,  in  accordance  with 
the  view  certain  physicists  take  of  the  principle  of 
Carnot.  We  shall  see,  for  example,  that  in  the  kinetic 
theory  we  are  led  to  admit  that,  after  waiting  suffi- 
ciently long,  we  can  witness  the  return  of  the  various 
states  through  which  a  mass  of  gas,  for  example,  has 
passed  in  its  series  of  transformations. 

If  we  keep  to  the  present  era,  evolution  has  a 
fixed  direction — that  which  leads  to  an  increase  of 
entropy ;  and  it  is  possible  to  enquire,  in  any  given 
system  to  what  physical  manifestations  this  increase 
corresponds.  We  note  that  kinetic,  potential, 
electrical,  and  chemical  forms  of  energy  have  a 
great  tendency  to  transform  themselves  into  calorific 
energy.  A  chemical  reaction,  for  example,  gives  out 
energy;  but  if  the  reaction  is  not  produced  under 
very  special  conditions,  this  energy  immediately 
passes  into  the  calorific  form.  This  is  so  true,  that 
chemists  currently  speak  of  the  heat  given  out  by 


PKINCIPLES  83 

reactions  instead  of  regarding  the  energy  disengaged 
in  general. 

In  all  these  transformations  the  calorific  energy 
obtained  has  not,  from  a  practical  point  of  view, 
the  same  value  at  which  it  started.  One  cannot,  in 
fact,  according  to  the  principle  of  Carnot,  transform 
it  integrally  into  mechanical  energy,  since  the  heat 
possessed  by  a  body  can  only  yield  work  on  condi- 
tion that  a  part  of  it  falls  on  a  body  with  a  lower 
temperature.  Thus  appears  the  idea  that  energies 
which  exchange  with  each  other  and  correspond 
to  equal  quantities  have  not  the  same  qualitative 
value.  Form  has  its  importance,  and  there  are 
persons  who  prefer  a  golden  louis  to  four  pieces  of 
five  francs.  The  principle  of  Carnot  would  thus 
lead  us  to  consider  a  certain  classification  of  energies, 
and  would  show  us  that,  in  the  transformations 
possible,  these  energies  always  tend  to  a  sort  of 
diminution  of  quality — that  is,  to  a  degradation. 

It  would  thus  reintroduce  an  element  of  differen- 
tiation of  which  it  seems  very  difficult  to  give  a 
mechanical  explanation.  Certain  philosophers  and 
physicists  see  in  this  fact  a  reason  which  condemns 
a  priori  all  attempts  made  to  give  a  mechanical 
explanation  of  the  principle  of  Carnot. 

It  is  right,  however,  not  to  exaggerate  the  import- 
ance that  should  be  attributed  to  the  phrase  de- 
graded energy.  If  the  heat  is  not  equivalent  to  the 
work,  if  heat  at  99°  is  not  equivalent  to  heat  at  100°, 


84    THE  NEW   PHYSICS  AND  ITS  EVOLUTION 

that  means  that  we  cannot  in  practice  construct  an 
engine  which  shall  transform  all  this  heat  into 
work,  or  that,  for  the  same  cold  source,  the  output 
is  greater  when  the  temperature  of  the  hot  source 
is  higher  ;  but  if  it  were  possible  that  this  cold 
source  had  itself  the  temperature  of  absolute  zero, 
the  whole  heat  would  reappear  in  the  form  of  work. 
The  case  here  considered  is  an  ideal  and  extreme 
case,  and  we  naturally  cannot  realize  it ;  but  this 
consideration  suffices  to  make  it  plain  that  the 
classification  of  energies  is  a  little  arbitrary  and 
depends  more,  perhaps,  on  the  conditions  in  which 
mankind  lives  than  on  the  inmost  nature  of  things. 

In  fact,  the  attempts  which  have  often  been 
made  to  refer  the  principle  of  Carnot  to  mechanics 
have  not  given  convincing  results.  It  has  nearly 
always  been  necessary  to  introduce  into  the  attempt 
some  new  hypothesis  independent  of  the  funda- 
mental hypotheses  of  ordinary  mechanics,  and  equiva- 
lent, in  reality,  to  one  of  the  postulates  on  which 
the  ordinary  exposition  of  the  second  law  of  thermo- 
dynamics is  founded.  Helmholtz,  in  a  justly 
celebrated  theory,  endeavoured  to  fit  the  principle 
of  Carnot  into  the  principle  of  least  action;  but 
the  difficulties  regarding  the  mechanical  interpre- 
tation of  the  ir reversibility  of  physical  phenomena 
remain  entire.  Looking  at  the  question,  however, 
from  the  point  of  view  at  which  the  partisans  of  the 
kinetic  theories  of  matter  place  themselves,  the  prin- 


PRINCIPLES  85 

ciple  is  viewed  in  a  new  aspect.  Gibbs  and  after- 
wards Boltzmann  and  Professor  Planck  have  put 
forward  some  very  interesting  ideas  on  this  subject. 
By  following  the  route  they  have  traced,  we  come 
to  consider  the  principle  as  pointing  out  to  us  that 
a  given  system  tends  towards  the  configuration 
presented  by  the  maximum  probability,  and,  numeri- 
cally, the  entropy  would  even  be  the  logarithm  of 
this  probability.  Thus  two  different  gaseous  masses, 
enclosed  in  two  separate  receptacles  which  have  just 
been  placed  in  communication,  diffuse  themselves 
one  through  the  other,  and  it  is  highly  improbable 
that,  in  their  mutual  shocks,  both  kinds  of  mole- 
cules should  take  a  distribution  of  velocities  which 
reduce  them  by  a  spontaneous  phenomenon  to  the 
initial  state. 

We  should  have  to  wait  a  very  long  time  for  so 
extraordinary  a  concourse  of  circumstances,  but,  in 
strictness,  it  would  not  be  impossible.  The  principle 
would  only  be  a  law  of  probability.  Yet  this  pro- 
bability is  all  the  greater  the  more  considerable  is 
the  number  of  molecules  itself.  In  the  phenomena 
habitually  dealt  with,  this  number  is  such  that, 
practically,  the  variation  of  entropy  in  a  constant 
sense  takes,  so  to  speak,  the  character  of  absolute 
certainty. 

But  there  may  be  exceptional  cases  where  the 
complexity  of  the  system  becomes  insufficient  for 
the  application  of  the  principle  of  Carnot ; — as 


86    THE  NEW  PHYSICS  AND  ITS   EVOLUTION 

in  the  case  of  the  curious  movements  of  small 
particles  suspended  in  a  liquid  which  are  known 
by  the  name  of  Brownian  movements  and  can  be 
observed  under  the  microscope.  The  agitation  here 
really  seems,  as  M.  Gouy  has  remarked,  to  be  pro- 
duced and  continued  indefinitely,  regardless  of  any 
difference  in  temperature ;  and  we  seem  to  witness 
the  incessant  motion,  in  an  isothermal  medium,  of  the 
particles  which  constitute  matter.  Perhaps,  however, 
we  find  ourselves  already  in  conditions  where  the 
too  great  simplicity  of  the  distribution  of  the  mole- 
cules deprives  the  principle  of  its  value. 

M.  Lippmann  has  in  the  same  way  shown  that, 
on  the  kinetic  hypothesis,  it  is  possible  to  construct 
such  mechanisms  that  we  can  so  take  cognizance 
of  molecular  movements  that  vis  viva  can  be  taken 
from  them.  The  mechanisms  of  M.  Lippmann  are 
not,  like  the  celebrated  apparatus  at  one  time  de- 
vised by  Maxwell,  purely  hypothetical.  They  do 
not  suppose  a  partition  with  a  hole  impossible  to  be 
bored  through  matter  where  the  molecular  spaces 
would  be  larger  than  the  hole  itself.  They  have 
finite  dimensions.  Thus  M.  Lippmann  considers  a 
vase  full  of  oxygen  at  a  constant  temperature.  In 
the  interior  of  this  vase  is  placed  a  small  copper  ring, 
and  the  whole  is  set  in  a  magnetic  field.  The 
oxygen  molecules  are,  as  we  know,  magnetic,  and 
when  passing  through  the  interior  of  the  ring  they 
produce  in  this  ring  an  induced  current.  During 


PRINCIPLES  87 

this  time,  it  is  true,  other  molecules  emerge  from 
the  space  enclosed  by  the  circuit ;  but  the  two  effects 
do  not  counterbalance  each  other,  and  the  result- 
ing current  is  maintained.  There  is  elevation  of 
temperature  in  the  circuit  in  accordance  with  Joule's 
law;  and  this  phenomenon,  under  such  conditions, 
is  incompatible  with  the  principle  of  Carnot. 

It  is  possible — and  that,  I  think,  is  M.  Lippmann's 
idea — to  draw  from  his  very  ingenious  criticism  an 
objection  to  the  kinetic  theory,  if  we  admit  the 
absolute  value  of  the  principle;  but  we  may  also 
suppose  that  here  again  we  are  in  presence  of  a 
system  where  the  prescribed  conditions  diminish  the 
complexity  and  render  it,  consequently,  less  probable 
that  the  evolution  is  always  effected  in  the  same 
direction. 

In  whatever  way  you  look  at  it,  the  principle 
of  Carnot  furnishes,  in  the  immense  majority  of 
cases,  a  very  sure  guide  in  which  physicists  continue 
to  have  the  most  entire  confidence. 

§  4.  THERMODYNAMICS 

To  apply  the  two  fundamental  principles  of 
thermodynamics,  various  methods  may  be  employed, 
equivalent  in  the  main,  but  presenting  as  the  cases 
vary  a  greater  or  less  convenience. 

In  recording,  with  the  aid  of  the  two  quantities, 
energy  and  entropy,  the  relations  which  translate 
analytically  the  two  principles,  we  obtain  two 


88    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

relations  between  the  coefficients  which  occur  in  a 
given  phenomenon;  but  it  may  be  easier  and  also 
more  suggestive  to  employ  various  functions  of 
these  quantities.  In  a  memoir,  of  which  some 
extracts  appeared  as  early  as  1869,  a  modest  scholar, 
M.  Massieu,  indicated  in  particular  a  remarkable 
function  which  he  termed  a  characteristic  function, 
and  by  the  employment  of  which  calculations  are 
simplified  in  certain  cases. 

In  the  same  way  J.  W.  Gibbs,  in  1875  and  1878, 
then  Helmholtz  in  1882,  and,  in  France,  M.  Duhem, 
from  the  year  1886  onward,  have  published  works,  at 
first  ill  understood,  of  which  the  renown  was,  how- 
ever, considerable  in  the  sequel,  and  in  which  they 
made  use  of  analogous  functions  under  the  names  of 
available  energy,  free  energy,  or  internal  thermo- 
dynamic  potential.  The  magnitude  thus  designated, 
attaching,  as  a  consequence  of  the  two  principles, 
to  all  states  of  the  system,  is  perfectly  determined 
when  the  temperature  and  other  normal  variables 
are  known.  It  allows  us,  by  calculations  often  very 
easy,  to  fix  the  conditions  necessary  and  sufficient 
for  the  maintenance  of  the  system  in  equilibrium 
by  foreign  bodies  taken  at  the  same  temperature  as 
itself. 

One  may  hope  to  constitute  in  this  way,  as 
M.  Duhem  in  a  long  and  remarkable  series  of  opera- 
tions has  specially  endeavoured  to  do,  a  sort  of 
general  mechanics  which  will  enable  questions  of 


PRINCIPLES  89 

statics  to  be  treated  with  accuracy,  and  all  the  con- 
ditions of  equilibrium  of  the  system,  including  the 
calorific  properties,  to  be  determined.  Thus,  ordinary 
statics  teaches  us  that  a  liquid  with  its  vapour  on 
the  top  forms  a  system  in  equilibrium,  if  we  apply  to 
the  two  fluids  a  pressure  depending  on  temperature 
alone.  Thermodynamics  will  furnish  us,  in  addition, 
with  the  expression  of  the  heat  of  vaporization  and 
of  the  specific  heats  of  the  two  saturated  fluids. 

This  new  study  has  given  us  also  most  valuable 
information  on  compressible  fluids  and  on  the  theory 
of  elastic  equilibrium.  Added  to  certain  hypo- 
theses on  electric  or  magnetic  phenomena,  it  gives 
a  coherent  whole  from  which  can  be  deduced  the 
conditions  of  electric  or  magnetic  equilibrium ;  and 
it  illuminates  with  a  brilliant  light  the  calorific 
laws  of  electrolytic  phenomena. 

But  the  most  indisputable  triumph  of  this  thermo- 
dynamic  statics  is  the  discovery  of  the  laws  which 
regulate  the  changes  of  physical  state  or  of 
chemical  constitution.  J.  W.  Gibbs  was  the  author 
of  this  immense  progress.  His  memoir,  now  cele- 
brated, on  "the  equilibrium  of  heterogeneous  sub- 
stances," concealed  in  1876  in  a  review  at  that  time 
of  limited  circulation,  and  rather  heavy  to  read, 
seemed  only  to  contain  algebraic  theorems  appli* 
cable  with  difficulty  to  reality.  It  is  known  that 
Helmholtz  independently  succeeded,  a  few  years 
later,  in  introducing  thermodynamics  into  the 


90    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

domain  of  chemistry  by  his  conception  of  the 
division  of  energy  into  free  and  into  bound  energy : 
the  first,  capable  of  undergoing  all  transformations, 
and  particularly  of  transforming  itself  into  external 
action;  the  second,  on  the  other  hand,  bound,  and 
only  manifesting  itself  by  giving  out  heat.  When 
we  measure  chemical  energy,  we  ordinarily  let  it 
fall  wholly  into  the  calorific  form;  but,  in  reality, 
it  itself  includes  both  parts,  and  it  is  the  variation 
of  the  free  energy  and  not  that  of  the  total 
energy  measured  by  the  integral  disengagement  of 
heat,  the  sign  of  which  determines  the  direction 
in  which  the  reactions  are  effected. 

But  if  the  principle  thus  enunciated  by  Helmholtz 
as  a  consequence  of  the  laws  of  thermodynamics  is 
at  bottom  identical  with  that  discovered  by  Gibbs, 
it  is  more  difficult  of  application  and  is  presented 
under  a  more  mysterious  aspect.  It  was  not  until 
M.  Van  der  Waals  exhumed  the  memoir  of  Gibbs, 
when  numerous  physicists  or  chemists,  most  of  them 
Dutch — Professor  Van  t'Hoff,  Bakhius  Eoozeboom, 
and  others — utilized  the  rules  set  forth  in  this 
memoir  for  the  discussion  of  the  most  complicated 
chemical  reactions,  that  the  extent  of  the  new  laws 
was  fully  understood. 

The  chief  rule  of  Gibbs  is  the  one  so  celebrated 
at  the  present  day  under  the  name  of  the  Phase 
Law.  We  know  that  by  phases  are  designated  the 
homogeneous  substances  into  which  a  system  is 


PEINCIPLES  91 

divided  ;  thus  carbonate  of  lime,  lime,  and  carbonic 
acid  gas  are  the  three  phases  of  a  system  which 
comprises  Iceland  spar  partially  dissociated  into 
lime  and  carbonic  acid  gas.  The  number  of  phases 
added  to  the  number  of  independent  components — 
that  is  to  say,  bodies  whose  mass  is  left  arbitrary 
by  the  chemical  formulas  of  the  substances  entering 
into  the  reaction — fixes  the  general  form  of  the  law 
of  equilibrium  of  the  system;  that  is  to  say,  the 
number  of  quantities  which,  by  their  variations 
(temperature  and  pressure),  would  be  of  a  nature  to 
modify  its  equilibrium  by  modifying  the  constitution 
of  the  phases. 

Several  authors,  M.  Eaveau  in  particular,  have 
indeed  given  very  simple  demonstrations  of  this 
law  which  are  not  based  on  thermodynamics; 
but  thermodynamics,  which  led  to  its  discovery, 
continues  to  give  it  its  true  scope.  Moreover,  it 
would  not  suffice  merely  to  determine  quantitatively 
those  laws  of  which  it  makes  known  the  general 
form.  We  must,  if  we  wish  to  penetrate  deeper  into 
details,  particularize  the  hypothesis,  and  admit,  for 
instance,  with  Gibbs  that  we  are  dealing  with 
perfect  gases  ;  while,  thanks  to  thermodynamics,  we 
can  constitute  a  complete  theory  of  dissociation 
which  leads  to  formulas  in  complete  accord  with 
the  numerical  results  of  the  experiment.  We  can 
thus  follow  closely  all  questions  concerning  the 
displacements  of  the  equilibrium,  and  find  a  relation 


92    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

of  the  first  importance  between  the  masses  of  the 
bodies  which  react  in  order  to  constitute  a  system 
in  equilibrium. 

The  statics  thus  constructed  constitutes  at  the 
present  day  an  important  edifice  to  be  henceforth 
classed  amongst  historical  monuments.  Some 
theorists  even  wish  to  go  a  step  beyond.  They 
have  attempted  to  begin  by  the  same  means  a  more 
complete  study  of  those  systems  whose  state  changes 
from  one  moment  to  another.  This  is,  moreover, 
a  study  which  is  necessary  to  complete  satisfactorily 
the  study  of  equilibrium  itself ;  for  without  it  grave 
doubts  would  exist  as  to  the  conditions  of  stability, 
and  it  alone  can  give  their  true  meaning  to  questions 
relating  to  displacements  of  equilibrium. 

The  problems  with  which  we  are  thus  confronted 
are  singularly  difficult.  M.  Duhem  has  given  us 
many  excellent  examples  of  the  fecundity  of  the 
method;  but  if  thermodynamic  statics  may  be 
considered  definitely  founded,  it  cannot  be  said 
that  the  general  dynamics  of  systems,  considered  as 
the  study  of  thermal  movements  and  variations,  are 
yet  as  solidly  established. 

§  5.  ATOMISM 

It  may  appear  singularly  paradoxical  that,  in  a 
chapter  devoted  to  general  views  on  the  principles 
of  physics,  a  few  words  should  be  introduced  on  the 
atomic  theories  of  matter. 


PRINCIPLES  93 

Very  often,  in  fact,  what  is  called  the  physics  of 
principles  is  set  in  opposition  to  the  hypotheses  on 
the  constitution  of  matter,  particularly  to  atomic 
theories.  I  have  already  said  that,  abandoning  the 
investigation  of  the  unfathomable  mystery  of  the 
constitution  of  the  Universe,  some  physicists  think 
they  may  find,  in  certain  general  principles,  sufficient 
guides  to  conduct  them  across  the  physical  world. 
But  I  have  also  said,  in  examining  the  history  of 
those  principles,  that  if  they  are  to-day  considered 
experimental  truths,  independent  of  all  theories 
relating  to  matter,  they  have,  in  fact,  nearly  all 
been  discovered  by  scholars  who  relied  on  molecular 
hypotheses :  and  the  question  suggests  itself  whether 
this  is  mere  chance,  or  whether  this  chance  may  not 
be  ordained  by  higher  reasons. 

In  a  very  profound  work  which  appeared  a  few 
years  ago,  entitled  Essai  critique  sur  I'hypoth&se 
des  atomes,  M.  Hannequin,  a  philosopher  who  is 
also  an  erudite  scholar,  examined  the  part  taken  by 
atomism  in  the  history  of  science.  He  notes  that 
atomism  and  science  were  born,  in  Greece,  of  the 
same  problem,  and  that  in  modern  times  the  revival 
of  the  one  was  closely  connected  with  that  of  the 
other.  He  shows,  too,  by  very  close  analysis,  that 
the  atomic  hypothesis  is  essential  to  the  optics  of 
Fresnel  and  of  Cauchy ;  that  it  penetrates  into  the 
study  of  heat ;  and  that,  in  its  general  features,  it 
presided  at  the  birth  of  modern  chemistry  and  is 


94    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

linked  with  all  its  progress.  He  concludes  that  it  is, 
in  a  manner,  the  soul  of  our  knowledge  of  Nature,  and 
that  contemporary  theories  are  on  this  point  in  accord 
with  history :  'for  these  theories  consecrate  the  pre- 
ponderance of  this  hypothesis  in  the  domain  of  science. 

If  M.  Hannequin  had  not  been  prematurely  cut 
off  in  the  full  expansion  of  his  vigorous  talent,  he 
might  have  added  another  chapter  to  his  excellent 
book.  He  would  have  witnessed  a  prodigious  bud- 
ding of  atomistic  ideas,  accompanied,  it  is  true,  by 
wide  modifications  in  the  manner  in  which  the 
atom  is  to  be  regarded,  since  the  most  recent  theories 
make  material  atoms  into  centres  constituted  of 
atoms  of  electricity.  On  the  other  hand,  he  would 
have  found  in  the  bursting  forth  of  these  new 
doctrines  one  more  proof  in  support  of  his  idea  that 
science  is  indissolubly  bound  to  atomism. 

From  the  philosophical  point  of  view,  M. 
Hannequin,  examining  the  reasons  which  may  have 
called  these  links  into  being,  arrives  at  the  idea  that 
they  necessarily  proceed  from  the  constitution  of 
our  knowledge,  or,  perhaps,  from  that  of  Nature  itself. 
Moreover,  this  origin,  double  in  appearance,  is  single 
at  bottom.  Our  minds  could  not,  in  fact,  detach 
and  come  out  of  themselves  to  grasp  reality  and  the 
absolute  in  Nature.  According  to  the  idea  of 
Descartes,  it  is  the  destiny  of  our  minds  only  to 
take  hold  of  and  to  understand  that  which  proceeds 
from  them. 


PRINCIPLES  95 

Thus  atomism,  which  is,  perhaps,  only  an  appear- 
ance containing  even  some  contradictions,  is  yet  a 
well-founded  appearance,  since  it  conforms  to  the 
laws  of  our  minds  ;  and  this  hypothesis  is,  in  a  way, 
necessary. 

We  may  dispute  the  conclusions  of  M.  Hannequin, 
but  no  one  will  refuse  to  recognise,  as  he  does,  that 
atomic  theories  occupy  a  preponderating  part  in  the 
doctrines  of  physics;  and  the  position  which  they 
have  thus  conquered  gives  them,  in  a  way,  the  right 
of  saying  that  they  rest  on  a  real  principle.  It 
is  in  order  to  recognise  this  right  that  several 
physicists — M.  Langevin,  for  example — ask  that 
atoms  be  promoted  from  the  rank  of  hypotheses  to 
that  of  principles.  By  this  they  mean  that  the 
atomistic  ideas  forced  upon  us  by  an  almost  obligatory 
induction  based  on  very  exact  experiments,  enable 
us  to  co-ordinate  a  considerable  amount  of  facts,  to 
construct  a  very  general  synthesis,  and  to  foresee  a 
great  number  of  phenomena. 

It  is  of  moment,  moreover,  to  thoroughly  under- 
stand that  atomism  does  not  necessarily  set  up 
the  hypothesis  of  centres  of  attraction  acting  at  a 
distance,  and  it  must  not  be  confused  with  molecular 
physics,  which  has,  on  the  other  hand,  undergone  very 
serious  checks.  The  molecular  physics  greatly  in 
favour  some  fifty  years  ago  leads  to  such  complex 
representations  and  to  solutions  often  so  undeter- 
mined, that  the  most  courageous  are  wearied  with 


96    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

upholding  it  and  it  has  fallen  into  some  discredit.  It 
rested  on  the  fundamental  principles  of  mechanics 
applied  to  molecular  actions ;  and  that  was,  no  doubt, 
an  extension  legitimate  enough,  since  mechanics  is 
itself  only  an  experimental  science,  and  its  principles, 
established  for  the  movements  of  matter  taken  as  a 
whole,  should  not  be  applied  outside  the  domain 
which  belongs  to  them.  Atomism,  in  fact,  tends  more 
and  more,  in  modern  theories,  to  imitate  the  principle 
of  the  conservation  of  energy  or  that  of  entropy,  to 
disengage  itself  from  the  artificial  bonds  which 
attached  it  to  mechanics,  and  to  put  itself  forward 
as  an  independent  principle. 

Atomistic  ideas  also  have  undergone  evolution, 
and  this  slow  evolution  has  been  considerably 
quickened  under  the  influence  of  modern  dis- 
coveries. These  reach  back  to  the  most  remote 
antiquity,  and  to  follow  their  development  we 
should  have  to  write  the  history  of  human  thought 
which  they  have  always  accompanied  since  the  time 
of  Leucippus,  Democritus,  Epicurus,  and  Lucretius. 
The  first  observers  who  noticed  that  the  volume  of 
a  body  could  be  diminished  by  compression  or  cold, 
or  augmented  by  heat,  and  who  saw  a  soluble  solid 
body  mix  completely  with  the  water  which  dissolved 
it,  must  have  been  compelled  to  suppose  that  matter 
was  not  dispersed  continuously  throughout  the  space 
it  seemed  to  occupy.  They  were  thus  brought 
to  consider  it  discontinuous,  and  to  admit  that  a 


PRINCIPLES  97 

substance  having  the  same  composition  and  the  same 
properties  in  all  its  parts — in  a  word,  perfectly  homo- 
geneous— ceases  to  present  this  homogeneity  when 
considered  within  a  sufficiently  small  volume. 

Modern  experimenters  have  succeeded  by  direct 
experiments  in  placing  in  evidence  this  heterogeneous 
character  of  matter  when  taken  in  small  mass.  Thus, 
for  example,  the  superficial  tension,  which  is  constant 
for  the  same  liquid  at  a  given  temperature,  no  longer 
has  the  same  value  when  the  thickness  of  the  layer 
of  liquid  becomes  extremely  small.  Newton  noticed 
even  in  his  time  that  a  dark  zone  is  seen  to  form  on 
a  soap  bubble  at  the  moment  when  it  becomes  so  thin 
that  it  must  burst.  Professor  Eeinold  and  Sir  Arthur 
Kucker  have  shown  that  this  zone  is  no  longer  exactly 
spherical ;  and  from  this  we  must  conclude  that  the 
superficial  tension,  constant  for  all  thicknesses  above 
a  certain  limit,  commences  to  vary  when  the  thick- 
ness falls  below  a  critical  value,  which  these  authors 
estimate,  on  optical  grounds,  at  about  fifty  millionths 
of  a  millimetre. 

From  experiments  on  capillarity,  Prof.  Quincke 
has  obtained  similar  results  with  regard  to  layers  of 
solids.  But  it  is  not  only  capillary  properties  which 
allow  this  characteristic  to  be  revealed.  All  the 
properties  of  a  body  are  modified  when  taken  in  small 
mass  ;  M.  Meslin  proves  this  in  a  very  ingenious  way 
as  regards  optical  properties,  and  Mr  Vincent  in  re- 
spect of  electric  conductivity.  M.  Houllevigue,  who, 

7 


98     THE   NEW  PHYSICS  AND  ITS   EVOLUTION 

in  a  chapter  of  his  excellent  work,  Du  Laboratoire  CL 
r  Usine,  has  very  clearly  set  forth  the  most  interest- 
ing considerations  on  atomic  hypotheses,  has  recently 
demonstrated  that  copper  and  silver  cease  to  com- 
bine with  iodine  as  soon  as  they  are  present  in  a 
thickness  of  less  than  thirty  millionths  of  a  milli- 
metre. It  is  this  same  dimension  likewise  that  is 
possessed,  according  to  M.  Wiener,  by  the  smallest 
thicknesses  it  is  possible  to  deposit  on  glass.  These 
layers  are  so  thin  that  they  cannot  be  perceived, 
but  their  presence  is  revealed  by  a  change  in  the 
properties  of  the  light  reflected  by  them. 

Thus,  below  fifty  to  thirty  millionths  of  a  milli- 
metre the  properties  of  matter  depend  on  its 
thickness.  There  are  then,  no  doubt,  only  a  few 
molecules  to  be  met  with,  and  it  may  be  concluded, 
in  consequence,  that  the  discontinuous  elements  of 
bodies — that  is,  the  molecules — have  linear  dimen- 
sions of  the  order  of  magnitude  of  the  millionth  of  a 
millimetre.  Considerations  regarding  more  complex 
phenomena,  for  instance  the  phenomena  of  elec- 
tricity by  contact,  and  also  the  kinetic  theory  of 
gases,  bring  us  to  the  same  conclusion. 

The  idea  of  the  discontinuity  of  matter  forces 
itself  upon  us  for  many  other  reasons.  All  modern 
chemistry  is  founded  on  this  principle;  and  laws 
like  the  law  of  multiple  proportions,  introduce  an 
evident  discontinuity  to  which  we  find  analogies  in 
the  law  of  electrolysis.  The  elements  of  bodies  we 


PRINCIPLES  99 

are  thus  brought  to  regard  might,  as  regards  solids 
at  all  events,  be  considered  as  immobile;  but  this 
immobility  could  not  explain  the  phenomena  of  heat, 
and,  as  it  is  entirely  inadmissible  for  gases,  it  seems 
very  improbable  it  can  absolutely  occur  in  any  state. 
We  are  thus  led  to  suppose  that  these  elements 
are  animated  by  very  complicated  movements,  each 
one  proceeding  in  closed  trajectories  in  which  the 
least  variations  of  temperature  or  pressure  cause 
modifications. 

The  atomistic  hypothesis  shows  itself  remarkably 
fecund  in  the  study  of  phenomena  produced  in 
gases,  and  here  the  mutual  independence  of  the 
particles  renders  the  question  relatively  more  simple 
and,  perhaps,  allows  the  principles  of  mechanics 
to  be  more  certainly  extended  to  the  movements  of 
molecules. 

The  kinetic  theory  of  gases  can  point  to  un- 
questioned successes ;  and  the  idea  of  Daniel 
Bernouilli,  who,  as  early  as  1738,  considered  a 
gaseous  mass  to  be  formed  of  a  considerable  number 
of  molecules  animated  by  rapid  movements  of 
translation,  has  been  put  into  a  form  precise  enough 
for  mathematical  analysis,  and  we  have  thus  found 
ourselves  in  a  position  to  construct  a  really  solid 
foundation.  It  will  be  at  once  conceived,  on  this 
hypothesis,  that  pressure  is  the  resultant  of  the 
shocks  of  the  molecules  against  the  walls  of  the 
containing  vessel,  and  we  at  once  come  to  the 


ioo    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

demonstration  that  the  law  of  Mariotte  is  a  natural 
consequence  of  this  origin  of  pressure;  since,  if 
the  volume  occupied  by  a  certain  number  of  mole- 
cules is  doubled,  the  number  of  shocks  per  second 
on  each  square  centimetre  of  the  walls  becomes 
half  as  much.  But  if  we  attempt  to  carry  this 
further,  we  find  ourselves  in  presence  of  a  serious 
difficulty.  It  is  impossible  to  mentally  -follow 
every  one  of  the  many  individual  molecules  which 
compose  even  a  very  limited  mass  of  gas.  The  path 
followed  by  this  molecule  may  be  every  instant 
modified  by  the  chance  of  running  against  another, 
or  by  a  shock  which  may  make  it  rebound  in 
another  direction. 

The  difficulty  would  be  insoluble  if  chance  had  not 
laws  of  its  own.  It  was  Maxwell  who  first  thought 
of  introducing  into  the  kinetic  theory  the  calcula- 
tion of  probabilities.  Willard  Gibbs  and  Boltzmann 
later  on  developed  this  idea,  and  have  founded  a 
statistical  method  which  does  not,  perhaps,  give 
absolute  certainty,  but  which  is  certainly  most 
interesting  and  curious.  Molecules  are  grouped  in 
such  a  way  that  those  belonging  to  the  same  group 
may  be  considered  as  having  the  same  state  of 
movement ;  then  an  examination  is  made  of  the 
number  of  molecules  in  each  group,  and  what  are 
the  changes  in  this  number  from  one  moment  to 
another.  It  is  thus  often  possible  to  determine  the 
part  which  the  different  groups  have  in  the  total 


properties  of  the  system  and  in  the  phenomena 
which  may  occur. 

Such  a  method,  analogous  to  the  one  employed  by 
statisticians  for  following  the  social  phenomena  in  a 
population,  is  all  the  more  legitimate  the  greater  the 
number  of  individuals  counted  in  the  averages;  now, 
the  number  of  molecules  contained  in  a  limited  space 
— for  example,  in  a  centimetre  cube  taken  in  normal 
conditions — is  such  that  no  population  could  ever 
attain  so  high  a  figure.  All  considerations,  those 
we  have  indicated  as  well  as  ofrhers  which  might  be 
invoked  (for  example,  the  recent  researches  of  M. 
Spring  on  the  limit  of  visibility  of  fluorescence),  give 
this  result: — that  there  are,  in  this  space,  some 
twenty  thousand  millions  of  molecules.  Each  of 
these  must  receive  in  the  space  of  a  millimetre  about 
ten  thousand  shocks,  and  be  ten  thousand  times  thrust 
out  of  its  course.  The  free  path  of  a  molecule  is 
then  very  small,  but  it  can  be  singularly  augmented 
by  diminishing  the  number  of  them.  Tait  and 
Dewar  have  calculated  that,  in  a  good  modern 
vacuum,  the  length  of  the  free  path  of  the  remaining 
molecules  not  taken  away  by  the  air-pump  easily 
reaches  a  few  centimetres. 

By  developing  this  theory,  we  come  to  consider 
that,  for  a  given  temperature,  every  molecule  (and 
even  every  individual  particle,  atom,  or  ion)  which 
takes  part  in  the  movement  has,  on  the  average, 
the  same  kinetic  energy  in  every  body,  and  that  this 


IC2    THE  -NEW    PHFSIOS  AND  ITS  EVOLUTION 

energy  is  proportional  to  the  absolute  temperature ; 
so  that  it  is  represented  by  this  temperature  multi- 
plied by  a  constant  quantity  which  is  a  universal 
constant. 

This  result  is  not  an  hypothesis  but  a  very  great 
probability.  This  probability  increases  when  it  is 
noted  that  the  same  value  for  the  constant  is  met 
with  in  the  study  of  very  varied  phenomena;  for 
example,  in  certain  theories  on  radiation.  Know- 
ing the  mass  and  energy  of  a  molecule,  it  is 
easy  to  calculate  its  speed  ;  and  we  find  that  the 
average  speed  is  about  400  metres  per  second  for 
carbonic  anhydride,  500  for  nitrogen,  and  1850 
for  hydrogen  at  0°  C.  and  at  ordinary  pressure.  I 
shall  have  occasion,  later  on,  to  speak  of  much  more 
considerable  speeds  than  these  as  animating  other 
particles. 

The  kinetic  theory  has  permitted  the  diffusion  of 
gases  to  be  explained,  and  the  divers  circumstances 
of  the  phenomenon  to  be  calculated.  It  has  allowed 
us  to  show,  as  M.  Brillouin  has  done,  that  the  co- 
efficient of  diffusion  of  two  gases  does  not  depend 
on  the  proportion  of  the  gases  in  the  mixture ;  it 
gives  a  very  striking  image  of  the  phenomena  of 
viscosity  and  conductivity  ;  and  it  leads  us  to  think 
that  the  coefficients  of  friction  and  of  conductivity 
are  independent  of  the  density;  while  all  these 
previsions  have  been  verified  by  experiment.  It 
has  also  invaded  optics ;  and  by  relying  on  the 


PRINCIPLES  103 

principle  of  Doppler,  Professor  Michelson  has  suc- 
ceeded in  obtaining  from  it  an  explanation  of  the 
length  presented  by  the  spectral  rays  of  even  the 
most  rarefied  gases. 

But  however  interesting  are  these  results,  they 
would  not  have  sufficed  to  overcome  the  repugnance 
of  certain  physicists  for  speculations  which,  an 
imposing  mathematical  baggage  notwithstanding, 
seemed  to  them  too  hypothetical.  The  theory, 
moreover,  stopped  at  the  molecule,  and  appeared 
to  suggest  no  idea  which  could  lead  to  the  discovery 
of  the  key  to  the  phenomena  where  molecules 
exercise  a  mutual  influence  on  each  other.  The 
kinetic  hypothesis,  therefore,  remained  in  some  dis- 
favour with  a  great  number  of  persons,  particularly 
in  France,  until  the  last  few  years,  when  all  the 
recent  discoveries  of  the  conductivity  of  gases  and  of 
the  new  radiations  came  to  procure  for  it  a  new  and 
luxuriant  efflorescence.  It  may  be  said  that  the 
atomistic  synthesis,  but  yesterday  so  decried,  is  to- 
day triumphant. 

The  elements  which  enter  into  the  earlier  kinetic 
theory,  and  which,  to  avoid  confusion,  should  be 
always  designated  by  the  name  of  molecules,  were 
not,  truth  to  say,  in  the  eyes  of  the  chemists,  the 
final  term  of  the  divisibility  of  matter.  It  is  well 
known  that,  to  them,  except  in  certain  particular 
bodies  like  the  vapour  of  mercury  and  argon,  the 
molecule  comprises  several  atoms,  and  that,  in  com- 


104    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

pound  bodies,  the  number  of  these  atoms  may  even 
be  fairly  considerable.  But  physicists  rarely  needed 
to  have  recourse  to  the  consideration  of  these  atoms. 
They  spoke  of  them  to  explain  certain  particularities 
of  the  propagation  of  sound,  and  to  enunciate  laws 
relating  to  specific  heats;  but,  in  general,  they 
stopped  at  the  consideration  of  the  molecule. 

The  present  theories  carry  the  division  much 
further.  I  shall  not  dwell  now  on  these  theories, 
since,  in  order  to  thoroughly  understand  them,  many 
other  facts  must  be  examined.  But  to  avoid  all 
confusion,  it  remains  understood  that,  contrary,  no 
doubt,  to  etymology,  but  in  conformity  with  present 
custom,  I  shall  continue  in  what  follows  to  call 
atoms  those  particles  of  matter  which  have  till  now 
been  spoken  of ;  these  atoms  being  themselves, 
according  to  modern  views,  singularly  complex 
edifices  formed  of  elements,  of  which  we  shall  have 
occasion  to  indicate  the  nature  later. 


CHAPTEK  IV 
THE   VARIOUS    STATES    OF    MATTER 

§  1.  THE  STATICS  OF  FLUIDS 

THE  division  of  bodies  into  gaseous,  liquid,  and  solid, 
and  the  distinction  established  for  the  same  sub- 
stance between  the  three  states,  retain  a  great 
importance  for  the  applications  and  usages  of  daily 
life,  but  have  long  since  lost  their  absolute  value 
from  the  scientific  point  of  view. 

So  far  as  concerns  the  liquid  and  gaseous  states 
particularly,  the  already  antiquated  researches  of 
Andrews  confirmed  the  ideas  of  Cagniard  de  la 
Tour  and  established  the  continuity  of  the  two 
states.  A  group  of  physical  studies  has  thus  been 
constituted  on  what  may  be  called  the  statics  of 
fluids,  in  which  we  examine  the  relations  existing 
between  the  pressure,  the  volume,  and  the  tempera- 
ture of  bodies,  and  in  which  are  comprised,  under  the 
term  fluid,  gases  as  well  as  liquids. 

These  researches  deserve  attention  by  their  interest 
and  the  generality  of  the  results  to  which  they 


io6    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

have  led.  They  also  give  a  remarkable  example  of 
the  happy  effects  which  may  be  obtained  by  the 
combined  employment  of  the  various  methods  of 
investigation  used  in  exploring  the  domain  of 
nature.  Thermodynamics  has,  in  fact,  allowed  us 
to  obtain  numerical  relations  between  the  various 
coefficients,  and  atomic  hypotheses  have  led  to 
the  establishment  of  one  capital  relation,  the 
characteristic  equation  of  fluids  ;  while,  on  the  other 
hand,  experiment  in  which  the  progress  made  in 
the  art  of  measurement  has  been  utilized,  has  fur- 
nished the  most  valuable  information  on  all  the 
laws  of  compressibility  and  dilatation. 

The  classical  work  of  Andrews  was  not  very 
wide.  Andrews  did  not  go  much  beyond  pressures 
close  to  the  normal  and  ordinary  temperatures. 
Of  late  years  several  very  interesting  and  peculiar 
cases  have  been  examined  by  MM.  Cailletet, 
Mathias,  Batelli,  Leduc,  P.  Chappuis,  and  other 
physicists.  Sir  W.  Eamsay  and  Mr  S.  Young 
have  made  known  the  isothermal  diagrams1  of  a 
certain  number  of  liquid  bodies  at  the  ordinary 
temperature.  They  have  thus  been  able,  while 
keeping  to  somewhat  restricted  limits  of  tempera- 
ture and  pressure,  to  touch  upon  the  most  im- 

1  By  isothermal  diagram  is  meant  the  pattern  or  complex 
formed  when  the  isothermal  lines  are  arranged  in  curves  of 
which  the  pressure  is  the  ordinate  and  the  volume  the 
abscissa. — ED. 


THE  VARIOUS   STATES  OF  MATTER         107 

portant  questions,  since  they  found  themselves  in 
the  region  of  the  saturation  curve  and  of  the  critical 
point. 

But  the  most  complete  and  systematic  body  of 
researches  is  due  to  M.  Amagat,  who  undertook  the 
study  of  a  certain  number  of  bodies,  some  liquid  and 
some  gaseous,  extending  the  scope  of  his  experiments 
so  as  to  embrace  the  different  phases  of  the  phenomena 
and  to  compare  together,  not  only  the  results  relating 
to  the  same  bodies,  but  also  those  concerning  different 
bodies  which  happen  to  be  in  the  same  conditions  of 
temperature  and  pressure,  but  in  very  different  con- 
ditions as  regards  their  critical  points. 

From  the  experimental  point  of  view,  M.  Amagat 
has  been  able,  with  extreme  skill,  to  conquer  the 
most  serious  difficulties.  He  has  managed  to 
measure  with  precision  pressures  amounting  to 
3000  atmospheres,  and  also  the  very  small  volumes 
then  occupied  by  the  fluid  mass  under  consideration. 
This  last  measurement,  which  necessitates  numerous 
corrections,  is  the  most  delicate  part  of  the  operation. 
These  researches  have  dealt  with  a  certain  number 
of  different  bodies.  Those  relating  to  carbonic  acid 
and  ethylene  take  in  the  critical  point.  Others,  on 
hydrogen  and  nitrogen,  for  instance,  are  very  ex- 
tended. Others,  again,  such  as  the  study  of  the 
compressibility  of  water,  have  a  special  interest,  on 
account  of  the  peculiar  properties  of  this  substance. 
M.  Amagat,  by  a  very  concise  discussion  of  the 


io8    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

experiments,  has  also  been  able  to  definitely  establish 
the  laws  of  compressibility  and  dilatation  of  fluids 
under  constant  pressure,  and  to  determine  the 
value  of  the  various  coefficients  as  well  as  their 
variations.  It  ought  to  be  possible  to  condense  all 
these  results  into  a  single  formula  representing  the 
volume,  the  temperature,  and  the  pressure.  Kankin 
and,  subsequently,  Kecknagel,  and  then  Him, 
formerly  proposed  formulas  of  that  kind ;  but  the 
most  famous,  the  one  which  first  appeared  to  contain 
in  a  satisfactory  manner  all  the  facts  which  experi- 
ments brought  to  light  and  led  to  the  production 
of  many  others,  was  the  celebrated  equation  of 
Van  der  Waals. 

Professor  Van  der  Waals  arrived  at  this  relation  by 
relying  upon  considerations  derived  from  the  kinetic 
theory  of  gases.  If  we  keep  to  the  simple  idea 
at  the  bottom  of  this  theory,  we  at  once  demon- 
strate that  the  gas  ought  to  obey  the  laws  of 
Mariotte  and  of  Gay-Lussac,  so  that  the  character- 
istic equation  would  be  obtained  by  the  statement 
that  the  product  of  the  number  which  is  the  measure 
of  the  volume  by  that  which  is  the  measure  of  the 
pressure  is  equal  to  a  constant  coefficient  multiplied 
by  the  degree  of  the  absolute  temperature.  But  to 
get  at  this  result  we  neglect  two  important  factors. 

We  do  not  take  into  account,  in  fact,  the  attraction 
which  the  molecules  must  exercise  on  each  other. 
Now,  this  attraction,  which  is  never  absolutely  non- 


THE  VARIOUS  STATES   OF   MATTER         109 

existent,  may  become  considerable  when  the  molecules 
are  drawn  closer  together ;  that  is  to  say,  when  the 
compressed  gaseous  mass  occupies  a  more  and  more 
restricted  volume.  On  the  other  hand,  we  assimilate 
the  molecules,  as  a  first  approximation,  to  material 
points  without  dimensions ;  in  the  evaluation  of  the 
path  traversed  by  each  molecule  no  notice  is  taken 
of  the  fact  that,  at  the  moment  of  the  shock,  their 
centres  of  gravity  are  still  separated  by  a  distance 
equal  to  twice  the  radius  of  the  molecule. 

M.  Van  der  Waals  has  sought  out  the  modifications 
which  must  be  introduced  into  the  simple  character- 
istic equation  to  bring  it  nearer  to  reality.  He  ex- 
tends to  the  case  of  gases  the  considerations  by  which 
Laplace,  in  his  famous  theory  of  capillarity,  reduced 
the  effect  of  the  molecular  attraction  to  a  perpendicular 
pressure  exercised  on  the  surface  of  a  liquid.  This 
leads  him  to  add  to  the  external  pressure,  that  due  to 
the  reciprocal  attractions  of  the  gaseous  particles. 
On  the  other  hand,  when  we  attribute  finite  dimen- 
sions to  these  particles,  we  must  give  a  higher  value 
to  the  number  of  shocks  produced  in  a  given  time, 
since  the  effect  of  these  dimensions  is  to  diminish 
the  mean  path  they  traverse  in  the  time  which 
elapses  between  two  consecutive  shocks. 

The  calculation  thus  pursued  leads  to  our  adding 
to  the  pressure  in  the  simple  equation  a  term  which 
is  designated  the  internal  pressure,  and  which  is  the 
quotient  of  a  constant  by  the  square  of  the  volume ; 


no    THE   NEW  PHYSICS  AND  ITS   EVOLUTION 

also  to  our  deducting  from  the  volume  a  constant 
which  is  the  quadruple  of  the  total  and  invariable 
volume  which  the  gaseous  molecules  would  occupy 
did  they  touch  one  another. 

The  experiments  fit  in  fairly  well  with  the  formula 
of  Van  der  Waals,  but  considerable  discrepancies 
occur  when  we  extend  its  limits,  particularly  when 
the  pressures  throughout  a  rather  wider  interval 
are  considered ;  so  that  other  and  rather  more 
complex  formulas,  on  which  there  is  no  advantage 
in  dwelling,  have  been  proposed,  and,  in  certain 
cases,  better  represent  the  facts. 

But  the  most  remarkable  result  of  M.  Van  der 
Waals'  calculations  is  the  discovery  of  corresponding 
states.  For  a  long  time  physicists  spoke  of  bodies 
taken  in  a  comparable  state.  Dalton,  for  example, 
pointed  out  that  liquids  have  vapour-pressures 
equal  to  the  temperatures  equally  distant  from  their 
boiling-point ;  but  that  if,  in  this  particular  property, 
liquids  were  comparable  under  these  conditions  of 
temperature,  as  regards  other  properties  the  paral- 
lelism was  no  longer  to  be  verified.  No  general 
rule  was  found  until  M.  Van  der  Waals  first 
enunciated  a  primary  law,  viz.,  that  if  the  pressure, 
the  volume,  and  the  temperature  are  estimated  by 
taking  as  units  the  critical  quantities,  the  constants 
special  to  each  body  disappear  in  the  characteristic 
equation,  which  thus  becomes  the  same  for  all 
fluids. 


THE   VARIOUS   STATES  OF   MATTER         in 

The  words  corresponding  states  thus  take  a 
perfectly  precise  signification.  Corresponding  states 
are  those  for  which  the  numerical  values  of  the 
pressure,  volume,  and  temperature,  expressed  by 
taking  as  units  the  values  corresponding  to  the 
critical  point,  are  equal ;  and,  ija^coires^orAdiag 
states  any  two  fluids  have  exactly  the  same 

properties. 

M.  Natanson,  and  subsequently  P.  Curie  and  M. 
Meslin,  have  shown  by  various  considerations  that 
the  same  result  may  be  arrived  at  by  choosing  units 
which  correspond  to  any  corresponding  states ;  it  has 
also  been  shown  that  the  theorem  of  corresponding 
states  in  no  way  implies  the  exactitude  of  Van  der 
Waals'  formula.  In  reality,  this  is  simply  due  to  the 
fact  that  the  characteristic  equation  only  contains 
three  constants. 

The  philosophical  importance  and  the  practical 
interest  of  the  discovery  nevertheless  remain  con- 
siderable. As  was  to  be  expected,  numbers  of  experi- 
menters have  sought  whether  these  consequences  are 
duly  verified  in  reality.  M.  Amagat,  particularly, 
has  made  use  for  this  purpose  of  a  most  original 
and  simple  method.  He  remarks  that,  in  all  its 
generality,  the  law  may  be  translated  thus :  If  the 
isothermal  diagrams  of  two  substances  be  drawn 
to  the  same  scale,  taking"  as  unit  of  volume  and 
of  pressure  the  values  of  the  critical  constants, 
the  two  diagrams  should  coincide ;  that  -is  to  say, 


ii2    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

their  superposition  should  present  the  aspect  of 
one  diagram  appertaining  to  a  single  substance. 
Further,  if  we  possess  the  diagrams  of  two  bodies 
drawn  to  any  scales  and  referable  to  any  units  what- 
ever, as  the  changes  of  units  mean  changes  in  the 
scale  of  the  axes,  we  ought  to  make  one  of  the 
diagrams  similar  to  the  other  by  lengthening  or 
shortening  it  in  the  direction  of  one  of  the  axes. 
M.  Amagat  then  photographs  two  isothermal 
diagrams,  leaving  one  fixed,  but  arranging  the  other 
so  that  it  may  be  free  to  turn  round  each  axis  of 
the  co-ordinates ;  and  by  projecting,  by  means  of  a 
magic  lantern,  the  second  on  the  first,  he  arrives  in 
certain  cases  at  an  almost  complete  coincidence. 

This  mechanical  means  of  proof  thus  dispenses  with 
laborious  calculations,  but  its  sensibility  is  unequally 
distributed  over  the  different  regions  of  the  diagram. 
M.  Raveau  has  pointed  out  an  equally  simple  way  of 
verifying  the  law,  by  remarking  that  if  the  logarithms 
of  the  pressure  and  volume  are  taken  as  co-ordinates, 
the  co-ordinates  of  two  corresponding  points  differ 
by  two  constant  quantities,  and  the  corresponding 
curves  are  identical. 

From  these  comparisons,  and  from  other  im- 
portant researches,  among  which  should  be  particu- 
larly mentioned  those  of  Mr  S.  Young  and  M. 
Mathias,  it  results  that  the  laws  of  corresponding 
states  have  not,  unfortunately,  the  degree  of  gener- 
ality which  we  at  first  attributed  to  them,  but  that 


THE  VARIOUS  STATES  OF  MATTER         113 

they  are  satisfactory  when  applied  to  certain  groups 
of  bodies. 1 

If  in  the  study  of  the  statics  of  a  simple  fluid  the 
experimental  results  are  already  complex,  we  ought 
to  expect  much  greater  difficulties  when  we  come 
to  deal  with  mixtures;  still  the  problem  has  been 
approached,  and  many  points  are  already  cleared  up. 

Mixed  fluids  may  first  of  all  be  regarded  as 
composed  of  a  large  number  of  invariable  particles. 
In  this  particularly  simple  case  M.  Van  der  Waals 
has  established  a  characteristic  equation  of  the  mix- 
tures which  is  founded  on  mechanical  considerations. 
Various  verifications  of  this  formula  have  been 
effected,  and  it  has,  in  particular,  been  the  object  of 
very  important  remarks  by  M.  Daniel  Berthelot. 

It  is  interesting  to  note  that  thermodynamics 
seems  powerless  to  determine  this  equation,  for  it 
does  not  trouble  itself  about  the  nature  of  the 
bodies  obedient  to  its  laws;  but,  on  the  other  hand, 
it  intervenes  to  determine  the  properties  of  co- 
existing phases.  If  we  examine  the  conditions  of 
equilibrium  of  a  mixture  which  is  not  subjected  to 
external  forces,  it  will  be  demonstrated  that  the 
distribution  must  come  back  to  a  juxtaposition  of 

1  Mr  Preston  thus  puts  it :  "  The  law  [of  corresponding  states] 
seems  to  be  not  quite,  but  very  nearly  true  for  these  substances 
[i.e.  the  halogen  derivatives  of  benzene]  ;  but  in  the  case  of 
the  other  substances  examined,  the  majority  of  these  general- 
izations were  either  only  roughly  true  or  altogether  departed 
from  "  (Theory  of  Heat,  London,  1904,  p.  514.)— ED. 

8 


ii4    THE  NEW  PHYSICS   AND  ITS  EVOLUTION 

homogeneous  phases  ;  in  a  given  volume,  matter  ought 
so  to  arrange  itself  that  the  total  sum  of  free  energy 
has  a  minimum  value.  Thus,  in  order  to  elucidate 
all  questions  relating  to  the  number  and  qualities  of 
the  phases  into  which  the  substance  divides  itself,  we 
are  led  to  regard  the  geometrical  surface  which  for  a 
given  temperature  represents  the  free  energy. 

I  am  unable  to  enter  here  into  the  detail  of  the 
questions  connected  with  the  theories  of  Gibbs, 
which  have  been  the  object  of  numerous  theoretical 
studies,  and  also  of  a  series,  ever  more  and  more 
abundant,  of  experimental  researches.  M.  Duhem, 
in  particular,  has  published,  on  the  subject,  memoirs 
of  the  highest  importance,  and  a  great  number 
of  experimenters,  mostly  scholars  working  in  the 
physical  laboratory  of  Leyden  under  the  guidance 
of  the  Director,  Mr  Kamerlingh  Onnes,  have  endea- 
voured to  verify  the  anticipations  of  the  theory. 

We  are  a  little  less  advanced  as  regards  abnormal 
substances;  that  is  to  say,  those  composed  of  molecules, 
partly  simple  and  partly  complex, and  either  dissociated 
or  associated.  These  cases  must  naturally  be  governed 
by  very  complex  laws.  Eecent  researches  by  MM. 
Van  der  Waals,  Alexeif,  Eothmund,  Kunen,  Lehfeld, 
etc.,  throw,  however,  some  light  on  the  question. 

The  daily  more  numerous  applications  of  the  laws 
of  corresponding  states  have  rendered  highly  im- 
portant the  determination  of  the  critical  constants 
which  permit  these  states  to  be  defined.  In  the  case 


THE  VARIOUS  STATES  OF  MATTER         115 

of  homogeneous  bodies  the  critical  elements  have  a 
simple,  clear,  and  precise  sense ;  the  critical  tempera- 
ture is  that  of  the  single  isothermal  line  which 
presents  a  point  of  inflexion  at  a  horizontal  tangent ; 
the  critical  pressure  and  the  critical  volume  are  the 
two  co-ordinates  of  this  point  of  inflexion. 

The  three  critical  constants  may  be  determined, 
as  Mr  S.  Young  and  M.  Amagat  have  shown, 
by  a  direct  method  based  on  the  consideration 
of  the  saturated  states.  Eesults,  perhaps  more 
precise,  may  also  be  obtained  if  one  keeps  to  two 
constants  or  even  to  a  single  one — temperature,  for 
example — by  employing  various  special  methods. 
Many  others,  MM.  Cailletet  and  Colardeau,  M. 
Young,  M.  J.  Chappuis,  etc.,  have  proceeded  thus. 

The  case  of  mixtures  is  much  more  complicated. 
A  binary  mixture  has  a  critical  space  instead  of  a 
critical  point.  This  space  is  comprised  between  two 
extreme  temperatures,  the  lower  corresponding  to 
what  is  called  the  folding  point,  the  higher  to  that 
which  we  call  the  point  of  contact  of  the  mixture. 
Between  these  two  temperatures  an  isothermal 
compression  yields  a  quantity  of  liquid  which  in- 
creases, then  reaches  a  maximum,  diminishes,  and 
disappears.  This  is  the  phenomenon  of  retrograde 
condensation.  We  may  say  that  the  properties 
of  the  critical  point  of  a  homogeneous  substance 
are,  in  a  way,  divided,  when  it  is  a  question  of  a 
binary  mixture,  between  the  two  points  mentioned. 


ii6    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

Calculation  has  enabled  M.  Van  der  Waals,  by  the 
application  of  his  kinetic  theories,  and  M.  Duhem, 
by  means  of  thermodynamics,  to  foresee  most  of  the 
results  which  have  since  been  verified  by  experiment. 
All  these  facts  have  been  admirably  set  forth  and 
systematically  co-ordinated  by  M.  Mathias,  who, 
by  his  own  researches,  moreover,  has  made  con- 
tributions of  the  highest  value  to  the  study  of 
questions  regarding  the  continuity  of  the  liquid  and 
gaseous  states. 

The  further  knowledge  of  critical  elements  has 
allowed  the  laws  of  corresponding  states  to  be  more 
closely  examined  in  the  case  of  homogeneous 
substances.  It  has  shown  that,  as  I  have  already 
said,  bodies  must  be  arranged  in  groups,  and  this  fact 
clearly  proves  that  the  properties  of  a  given  fluid  are 
not  determined  by  its  critical  constants  alone,  and 
that  it  is  necessary  to  add  to  them  some  other 
specific  parameters ;  M.  Mathias  and  M.  D.  Berthelot 
have  indicated  some  which  seem  to  play  a  consider- 
able part. 

It  results  also  from  this  that  the  characteristic 
equation  of  a  fluid  cannot  yet  be  considered  perfectly 
known.  Neither  the  equation  of  Van  der  Waals  nor 
the  more  complicated  formulas  which  have  been  pro- 
posed by  various  authors  are  in  perfect  conformity 
with  reality.  We  may  think  that  researches  of  this 
kind  will  only  be  successful  if  attention  is  concen- 
trated, not  only  on  the  phenomena  of  compressi- 


THE   VARIOUS   STATES    OF  MATTER         117 

bility  and  dilatation,  but  also  on  the  calorimetric 
properties  of  bodies.  Thermodynamics  indeed  estab- 
lishes relations  between  those  properties  and  other 
constants,  but  does  not  allow  everything  to  be 
foreseen. 

Several  physicists  have  effected  very  interesting 
calorimetric  measurements,  either,  like  M.  Perot,  in 
order  to  verify  Clapeyron's  formula  regarding  the 
heat  of  vaporization,  or  to  ascertain  the  values  of 
specific  heats  and  their  variations  when  the  tempera- 
ture or  the  pressure  happens  to  change.  M.  Mathias 
has  even  succeeded  in  completely  determining  the 
specific  heats  of  liquefied  gases  and  of  their  satu- 
rated vapours,  as  well  as  the  heat  of  internal  and 
external  vaporization. 

§  2.  THE  LIQUEFACTION  OF   GASES,  AND  THE    PRO- 
PERTIES OF   BODIES  AT  A   LOW  TEMPERATURE 

The  scientific  advantages  of  all  these  researches 
have  been  great,  and,  as  nearly  always  happens,  the 
practical  consequences  derived  from  them  have  also 
been  most  important.  It  is  owing  to  the  more 
complete  knowledge  of  the  general  properties  of 
fluids  that  immense  progress  has  been  made  these 
last  few  years  in  the  methods  of  liquefying  gases. 

From  a  theoretical  point  of  view  the  new  pro- 
cesses of  liquefaction  can  be  classed  in  two  categories. 
Linde's  machine  and  those  resembling  it  utilize,  as 
is  known,  expansion  without  any  notable  produc- 


ii8    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

tion  of  external  work.  This  expansion,  nevertheless, 
causes  a  fall  in  the  temperature,  because  the  gas  in 
the  experiment  is  not  a  perfect  gas,  and,  by  an 
ingenious  process,  the  refrigerations  produced  are 
made  cumulative. 

Several  physicists  have  proposed  to  employ  a 
method  whereby  liquefaction  should  be  obtained  by 
expansion  with  recuperable  external  work.  This 
method,  proposed  as  long  ago  as  1860  by  Siemens, 
would  offer  considerable  advantages.  Theoretically, 
the  liquefaction  would  be  more  rapid,  and  obtained 
much  more  economically ;  but  unfortunately  in  the 
experiment  serious  obstacles  are  met  with,  especially 
"from  the  difficulty  of  obtaining  a  suitable  lubricant 
under  intense  cold  for  those  parts  of  the  machine 
which  have  to  be  in  movement  if  the  apparatus  is 
to  work. 

M.  Claude  has  recently  made  great  progress  on 
this  point  by  the  use,  during  the  running  of  the 
machine,  of  the  ether  of  petrol,  which  is  uncongeal- 
able,  and  a  good  lubricant  for  the  moving  parts. 
When  once  the  desired  region  of  cold  is  reached, 
air  itself  is  used,  which  moistens  the  metals  but 
does  not  completely  avoid  friction ;  so  that  the 
results  would  have  remained  only  middling,  had 
not  this  ingenious  physicist  devised  a  new  improve- 
ment which  has  some  analogy  with  superheating  of 
steam  in  steam  engines.  He  slightly  varies  the 
initial  temperature  of  the  compressed  air  on  the 


THE  VARIOUS  STATES   OF   MATTER         119 

verge  of  liquefaction  so  as  to  avoid  a  zone  of 
deep  perturbations  in  the  properties  of  fluids, 
which  would  make  the  work  of  expansion  very 
feeble  and  the  cold  produced  consequently  slight. 
This  improvement,  simple  as  it  is  in  appearance, 
presents  several  other  advantages  which  immedi- 
ately treble  the  output. 

The  special  object  of  M.  Claude  was  to  obtain 
oxygen  in  a  practical  manner  by  the  actual  distil- 
lation of  liquid  air.  Since  nitrogen  boils  at  —194° 
and  oxygen  at  — 180-5°  C.,  if  liquid  air  be  evapor- 
ated, the  nitrogen  escapes,  especially  at  the  com- 
mencement of  the  evaporation,  while  the  oxygen 
concentrates  in  the  residual  liquid,  which  finally 
consists  of  pure  oxygen,  while  at  the  same  time  the 
temperature  rises  to  the  boiling-point  (  — 18O50  C.) 
of  oxygen.  But  liquid  air  is  costly,  and  if  one  were 
content  to  evaporate  it  for  the  purpose  of  collecting  a 
part  of  the  oxygen  in  the  residuum,  the  process  would 
have  a  very  poor  result  from  the  commercial  point  of 
view.  As  early  as  1892,  Mr  Parkinson  thought  of  im- 
proving the  output  by  recovering  the  cold  produced  by 
liquid  air  during  its  evaporation  ;  but  an  incorrect 
idea,  which  seems  to  have  resulted  from  certain  experi- 
ments of  Dewar — the  idea  that  the  phenomenon  of 
the  liquefaction  of  air  would  not  be,  owing  to  certain 
peculiarities,  the  exact  converse  of  that  of  vaporiza- 
tion— led  to  the  employment  of  very  imperfect 
apparatus.  M.  Claude,  however,  by  making  use  of 


120    THE  NEW  PHYSICS  AND  ITS   EVOLUTION 

a  method  which  he  calls  the  reversal l  method, 
obtains  a  complete  rectification  in  a  remarkably 
simple  manner  and  under  extremely  advantageous 
economic  conditions.  Apparatus,  of  surprisingly 
reduced  dimensions  but  of  great  efficiency,  is  now 
in  daily  work,  which  easily  enables  more  than  a 
thousand  cubic  metres  of  oxygen  to  be  obtained  at 
the  rate,  per  horse-power,  of  more  than  a  cubic 
metre  per  hour. 

It  is  in  England,  thanks  to  the  skill  of  Sir  James 
Dewar  and  his  pupils — thanks  also,  it  must  be  said, 
to  the  generosity  of  the  Royal  Institution,  which  has 
devoted  considerable  sums  to  these  costly  experiments 
— that  the  most  numerous  and  systematic  researches 
have  been  effected  on  the  production  of  intense  cold. 
I  shall  here  note  only  the  more  important  results, 
especially  those  relating  to  the  properties  of  bodies 
at  low  temperatures. 

Their  electrical  properties,  in  particular,  undergo 
some  interesting  modifications.  The  order  which 
metals  assume  in  point  of  conductivity  is  no  longer 
the  same  as  at  ordinary  temperatures.  Thus  at 
—  200°  C.  copper  is  a  better  conductor  than  silver. 
The  resistance  diminishes  with  the  temperature,  and, 
down  to  about  —200°,  this  diminution  is  almost 
linear,  and  it  would  seem  that  the  resistance  tends 
towards  zero  when  the  temperature  approaches  the 
absolute  zero.  But,  after  —  200°,  the  pattern  of  the 
1  Methode  avec  tetour  en  arriere. — ED. 


THE  VARIOUS   STATES   OF   MATTER         121 

curves  changes,  and  it  is  easy  to  foresee  that  at 
absolute  zero  the  resistivities  of  all  metals  would 
still  have,  contrary  to  what  was  formerly  supposed, 
a  notable  value.  Solidified  electrolytes  which,  at 
temperatures  far  below  their  fusion  point,  still 
retain  a  very  appreciable  conductivity,  become,  on 
the  contrary,  perfect  insulators  at  low  temperatures. 
Their  dielectric  constants  assume  relatively  high 
values.  MM.  Curie  and  Compan,  who  have  studied 
this  question  from  their  own  point  of  view,  have 
noted,  moreover,  that  the  specific  inductive  capacity 
changes  considerably  with  the  temperature. 

In  the  same  way,  magnetic  properties  have  been 
studied.  A  very  interesting  result  is  that  found  in 
oxygen :  the  magnetic  susceptibility  of  this  body  in- 
creases at  the  moment  of  liquefaction.  Nevertheless, 
this  increase,  which  is  enormous  (since  the  suscep- 
tibility becomes  sixteen  hundred  times  greater  than 
it  was  at  first),  if  we  take  it  in  connection  with 
equal  volumes,  is  much  less  considerable  if  taken  in 
equal  masses.  It  must  be  concluded  from  this 
fact  that  the  magnetic  properties  apparently  do  not 
belong  to  the  molecules  themselves,  but  depend  on 
their  state  of  aggregation. 

The  mechanical  properties  of  bodies  also  undergo 
important  modifications.  *  In  general,  their  cohesion 
is  greatly  increased,  and  the  dilatation  produced  by 
slight  changes  of  temperature  is  considerable.  Sir 
James  Dewar  has  effected  careful  measurements  of 


122    THE  NEW  PHYSICS   AND   ITS  EVOLUTION 

the  dilatation  of  certain  bodies  at  low  temperatures : 
for  example,  of  ice.  Changes  in  colour  occur,  and 
vermilion  and  iodide  of  mercury  pass  into  pale 
orange.  Phosphorescence  becomes  more  intense, 
and  most  bodies  of  complex'  structure — milk,  eggs, 
feathers,  cotton,  and  flowers — become  phosphorescent. 
The  same  is  the  case  with  certain  simple  bodies, 
such  as  oxygen,  which  is  transformed  into  ozone  and 
emits  a  white  light  in  the  process. 

Chemical  affinity  is  almost  put  an  end  to ; 
phosphorus  and  potassium  remain  inert  in  liquid 
oxygen.  It  should,  however,  be  noted,  and  this 
remark  has  doubtless  some  interest  for  the  theories 
of  photographic  action,  that  photographic  substances 
retain,  even  at  the  temperature  of  liquid  hydrogen,  a 
very  considerable  part  of  their  sensitiveness  to  light. 

Sir  James  Dewar  has  made  some  important  appli- 
cations of  low  temperatures  in  chemical  analysis  ;  he 
also  utilizes  them  to  create  a  vacuum.  His  researches 
have,  in  fact,  proved  that  the  pressure  of  air  con- 
gealed by  liquid  hydrogen  cannot  exceed  the 
millionth  of  an  atmosphere.  We  have,  then,  in  this 
process,  an  original  and  rapid  means  of  creating 
an  excellent  vacuum  in  apparatus  of  very  different 
kinds — a  means  which,  in  certain  cases,  may  be 
particularly  convenient.1 

1  Professor  Socldy,  in  a  paper  read  before  the  Royal  Society 
on  the  15th  November  1906,  warns  experimenters  against  vacua 
created  by  charcoal  cooled  in  liquid  air  (the  method  referred 


THE  VARIOUS   STATES   OF   MATTER         123 

Thanks  to  these  studies,  a  considerable  field  has 
been  opened  up  for  biological  research,  but  in  this, 
which  is  not  our  subject,  I  shall  notice  one  point 
only.  It  has  been  proved  that  vital  germs — bacteria, 
for  example — maybe  kept  for  seven  days  at  —  190°C. 
without  their  vitality  being  modified.  Phosphorescent 
organisms  cease,  it  is  true,  to  shine  at  the  tempera- 
ture of  liquid  air,  but  this  fact  is  simply  due  to  the 
oxidations  and  other  chemical  reactions  which  keep 
up  the  phosphorescence  being  then  suspended,  for 
phosphorescent  activity  reappears  so  soon  as  the 
temperature  is  again  sufficiently  raised.  An  im- 
portant conclusion  has  been  drawn  from  these 
experiments  which  affects  cosmogonical  theories: 
since  the  cold  of  space  could  not  kill  the  germs 
of  life,  it  is  in  no  way  absurd  to  suppose  that,  under 
proper  conditions,  a  germ  may  be  transmitted  from 
one  planet  to  another. 

Among  the  discoveries  made  with  the  new 
processes,  the  one  which  most  strikingly  interested 
public  attention  is  that  of  new  gases  in  the  atmo- 
sphere. We  know  how  Sir  William  Ramsay  and 
Dr  Travers  first  observed  by  means  of  the  spectro- 
scope the  characteristics  of  the  companions  of  argon 
in  the  least  volatile  part  of  the  atmosphere.  Sir 

to  in  the  text),  unless  as  much  of  the  air  as  possible  is  first 
removed  with  a  pump  and  replaced  by  some  argon-free  gas. 
According  to  him,  neither  helium  nor  argon  is  absorbed  by 
charcoal.  By  the  use  of  electrically-heated  calcium,  he  claims 
to  have  produced  an  almost  perfect  vacuum. — ED. 


124    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

James  Dewar  on  the  one  hand,  and  Sir  William 
Ramsay  on  the  other,  subsequently  separated  in 
addition  to  argon  and  helium,  crypton,  xenon,  and 
neon.  The  process  employed  consists  essentially  in 
first  solidifying  the  least  volatile  part  of  the  air  and 
then  causing  it  to  evaporate  with  extreme  slowness. 
A  tube  with  electrodes  enables  the  spectrum  of  the 
gas  in  process  of  distillation  to  be  observed.  In  this 
manner,  the  spectra  of  the  various  gases  may  be 
seen  following  one  another  in  the  inverse  order  of 
their  volatility.  All  these  gases  are  monoatomic, 
like  mercury ;  that  is  to  say,  they  are  in  the  most 
simple  state,  they  possess  no  internal  molecular  energy 
(unless  it  is  that  which  heat  is  capable  of  supplying), 
and  they  even  seem  to  have  no  chemical  energy. 
Everything  leads  to  the  belief  that  they  show  the 
existence  on  the  earth  of  an  earlier  state  of 
things  now  vanished.  It  may  be  supposed,  for 
instance,  that  helium  and  neon,  of  which  the 
molecular  mass  is  very  slight,  were  formerly  more 
abundant  on  our  planet ;  but  at  an  epoch  when  the 
temperature  of  the  globe  was  higher,  the  very 
speed  of  their  molecules  may  have  reached  a  con- 
siderable value,  exceeding,  for  instance,  eleven  kilo- 
metres per  second,  which  suffices  to  explain  why 
they  should  have  left  our  atmosphere.  Crypton  and 
neon,  which  have  a  density  four  times  greater  than 
oxygen,  may,  on  the  contrary,  have  partly  dis- 
appeared by  solution  at  the  bottom  of  the  sea,  where 


THE  VARIOUS  STATES   OF   MATTER         125 

it  is  not  absurd  to  suppose  that  considerable  quan- 
tities would  be  found  liquefied  at  great  depths.1 

It  is  probable,  moreover,  that  the  higher  regions 
of  the  atmosphere  are  not  composed  of  the  same  air 
as  that  around  us.  Sir  James  Dewar  points  out  that 
Dal  ton's  law  demands  that  every  gas  composing  the 
atmosphere  should  have,  at  all  heights  and  tempera- 
tures, the  same  pressure  as  if  it  were  alone,  the 
pressure  decreasing  the  less  quickly,  all  things  being 
equal,  as  its  density  becomes  less.  It  results  from 
this  that  the  temperature  becoming  gradually  lower 
as  we  rise  in  the  atmosphere,  at  a  certain  altitude 
there  can  no  longer  remain  any  traces  of  oxygen 
or  nitrogen,  which  no  doubt  liquefy,  and  the  atmo- 
sphere must  be  almost  exclusively  composed  of  the 
most  volatile  gases,  including  hydrogen,  which 
M.  A.  Gautier  has,  like  Lord  Eayleigh  and  Sir 
William  Eamsay,  proved  to  exist  in  the  air.  The 
spectrum  of  the  Aurora  borealis,  in  which  are  found 
the  lines  of  those  parts  of  the  atmosphere  which 
cannot  be  liquefied  in  liquid  hydrogen,  together 
with  the  lines  of  argon,  crypton,  and  xenon,  is 
quite  in  conformity  with  this  point  of  view.  It 
is,  however,  singular  that  it  should  be  the  spectrum 

1  Another  view,  viz.  that  these  inert  gases  are  a  kind  of 
waste  product  of  radioactive  changes,  is  also  gaining  ground. 
'The  discovery  of  the  radioactive  mineral  malacone,  which 
gives  off  both  helium  and  argon,  goes  to  support  this.  See 
Messrs  Ketchin  and  Winterson's  paper  on  the  subject  at  the 
Chemical  Society,  18th  October  1906.— ED. 


126    THE   NEW  PHYSICS  AND   ITS   EVOLUTION 

of  crypton,  that  is  to  say,  of  the  heaviest  gas  of 
the  group,  which  appears  most  clearly  in  the  upper 
regions  of  the  atmosphere. 

Among  the  gases  most  difficult  to  liquefy,  hydrogen 
has  been  the  object  of  particular  research  and  of 
really  quantitative  experiments.  Its  properties  in 
a  liquid  state  are  now  very  clearly  known.  Its 
boiling-point,  measured  with  a  helium  thermometer 
which  has  been  compared  with  thermometers  of 
oxygen  and  hydrogen,  is  —252°;  its  critical  tem- 
perature is  —241°  C. ;  its  critical  pressure,  15  atmo- 
spheres. It  is  four  times  lighter  than  water,  it 
does  not  present  any  absorption  spectrum,  and  its 
specific  heat  is  the  greatest  known.  It  is  not  a 
conductor  of  electricity.  Solidified  at  15°  absolute, 
it  is  far  from  reminding  one  by  its  aspect  of  a  metal ; 
it  rather  resembles  a  piece  of  perfectly  pure  ice,  and 
Dr  Travers  attributes  to  it  a  crystalline  structure. 
The  last  gas  which  has  resisted  liquefaction,  helium, 
has  recently  been  obtained  in  a  liquid  state ;  it 
appears  to  have  its  boiling-point  in  the  neighbour- 
hood of  6°  absolute.1 

§  3.  SOLIDS  AND  LIQUIDS 

The  interest  of  the  results  to  which  the  researches 
on  the  continuity  between  the  liquid  and  the  gaseous 
states  have  led  is  so  great,  that  numbers  of  scholars 

1  M.  Poincare  is  here  in  error.  Helium  has  never  been 
liquefied. — ED. 


THE  VAEIOUS  STATES  OF  MATTER        127 

have  naturally  been  induced  to  inquire  whether  some- 
thing analogous  might  not  be  found  in  the  case  of 
liquids  and  solids.  We  might  think  that  a  similar 
continuity  ought  to  be  there  met  with,  that  the 
universal  character  of  the  properties  of  matter  for- 
bade all  real  discontinuity  between  two  different 
states,  and  that,  in  truth,  the  solid  was  a  prolongation 
of  the  liquid  state. 

To  discover  whether  this  supposition  is  correct, 
it  concerns  us  to  compare  the  properties  of  liquids 
and  solids.  If  we  find  that  all  properties  are 
common  to  the  two  states  we  have  the  right 
to  believe,  even  if  they  presented  themselves  in 
different  degrees,  that,  by  a  continuous  series  of 
intermediary  bodies,  the  two  classes  might  yet  be 
connected.  If,  on  the  other  hand,  we  discover  that 
there  exists  in  these  two  classes  some  quality  of  a 
different  nature,  we  must  necessarily  conclude  that 
there  is  a  discontinuity  which  nothing  can  remove. 

The  distinction  established,  from  the  point  of 
view  of  daily  custom,  between  solids  and  liquids, 
proceeds  especially  from  the  difficulty  that  we  meet 
with  in  the  one  case,  and  the  facility  in  the  other, 
when  we  wish  to  change  their  form  temporarily 
or  permanently  by  the  action  of  mechanical  force. 
This  distinction  only  corresponds,  however,  in  reality, 
to  a  difference  in  the  value  of  certain  coefficients. 
It  is  impossible  to  discover  by  this  means  any 
absolute  characteristic  which  establishes  a  separation 


128    THE  NEW  PHYSICS   AND  ITS  EVOLUTION 

between  the  two  classes.  Modern  researches  prove 
this  clearly.  It  is  not  without  use,  in  order  to  well 
understand  them,  to  state  precisely  the  meaning  of 
a  few  terms  generally  rather  loosely  employed. 

If  a  conjunction  of  forces  acting  on  a  homogeneous 
material  mass  happens  to  deform  it  without  com- 
pressing or  dilating  it,  two  very  distinct  kinds  of 
reactions  may  appear  which  oppose  themselves 
to  the  effort  exercised.  During  the  time  of 
deformation,  and  during  that  time  only,  the  first 
make  their  influence  felt.  They  depend  essentially 
on  the  greater  or  less  rapidity  of  the  deformation, 
they  cease  with  the  movement,  and  could  not,  in  any 
case,  bring  the  body  back  to  its  pristine  state  of 
equilibrium.  The  existence  of  these  reactions  leads 
us  to  the  idea  of  viscosity  or  internal  friction. 

The  second  kind  of  reactions  are  of  a  different 
nature.  They  continue  to  act  when  the  deformation 
remains  stationary,  and,  if  the  external  forces  happen 
to  disappear,  they  are  capable  of  causing  the  body  to 
return  to  its  initial  form,  provided  a  certain  limit  has 
not  been  exceeded.  These  last  constitute  rigidity. 

At  first  sight  a  solid  body  appears  to  have  a 
finite  rigidity  and  an  infinite  viscosity ;  a  liquid,  on 
the  contrary,  presents  a  certain  viscosity,  but  no 
rigidity.  But  if  we  examine  the  matter  more  closely, 
beginning  either  with  the  solids  or  with  the  liquids, 
we  see  this  distinction  vanish. 

Tresca  showed  long  ago  that  internal  friction  is 


THE  VARIOUS  STATES  OF   MATTER         129 

not  infinite  in  a  solid;  certain  bodies  can,  so  to 
speak,  at  once  flow  and  be  moulded.  M.  W.  Spring 
has  given  many  examples  of  such  phenomena.  On  the 
other  hand,  viscosity  in  liquids  is  never  non-existent ; 
for  were  it  so  for  water,  for  example,  in  the  cele- 
brated experiment  effected  by  Joule  for  the  deter- 
mination of  the  mechanical  equivalent  of  the  caloric, 
the  liquid  borne  along  by  the  floats  would  slide 
without  friction  on  the  surrounding  liquid,  and  the 
work  done  by  movement  would  be  the  same  whether 
the  floats  did  or  did  not  plunge  into  the  liquid  mass. 

In  certain  cases  observed  long  ago  with  what  are 
called  pasty  bodies,  this  viscosity  attains  a  value 
almost  comparable  to  that  observed  by  M.  Spring 
in  some  solids.  Nor  does  rigidity  allow  us  to  estab- 
lish a  barrier  between  the  two  states.  Notwith- 
standing the  extreme  mobility  of  their  particles, 
liquids  contain,  in  fact,  vestiges  of  the  property 
which  we  formerly  wished  to  consider  the  special 
characteristic  of  solids. 

Maxwell  before  succeeded  in  rendering  the  exist- 
ence of  this  rigidity  very  probable  by  examin- 
ing the  optical  properties  of  a  deformed  layer  of 
liquid.  But  a  Russian  physicist,  M.  Schwedoff,  has 
gone  further,  and  has  been  able  by  direct  experi- 
ments to  show  that  a  sheath  of  liquid  set  between  two 
solid  cylinders  tends,  when  one  of  the  cylinders  is 
subjected  to  a  slight  rotation,  to  return  to  its  original 
position,  and  gives  a  measurable  torsion  to  a  thread 

9 


130    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

upholding  the  cylinder.  From  the  knowledge  of 
this  torsion  the  rigidity  can  he  deduced.  In  the  case 
of  a  solution  containing  J  per  cent,  of  gelatine,  it 
is  found  that  this  rigidity,  enormous  compared  with 
that  of  water,  is  still,  however,  one  trillion  eight 
hundred  and  forty  billion  times  less  than  that  of 
steel. 

This  figure,  exact  within  a  few  billions,  proves 
that  the  rigidity  is  very  slight,  but  exists ;  and 
that  suffices  for  a  characteristic  distinction  to  be 
founded  on  this  property.  In  a  general  way,  M. 
Spring  has  also  established  that  we  meet  in  solids, 
in  a  degree  more  or  less  marked,  with  the  properties 
of  liquids.  When  they  are  placed  in  suitable  con- 
ditions of  pressure  and  time,  they  flow  through 
orifices,  transmit  pressure  in  all  directions,  diffuse 
and  dissolve  one  into  the  other,  and  react  chemically 
on  each  other.  They  may  be  soldered  together  by 
compression  ;  by  the  same  means  alloys  may  be  pro- 
duced ;  and  further,  which  seems  to  clearly  prove 
that  matter  in  a  solid  state  is  not  deprived  of  all 
molecular  mobility,  it  is  possible  to  realise  suitable 
limited  reactions  and  equilibria  between  solid  salts, 
and  these  equilibria  obey  the  fundamental  laws  of 
thermodynamics. 

Thus  the  definition  of  a  solid  cannot  be  drawn 
from  its  mechanical  properties.  It  cannot  be  said, 
after  what  we  have  just  seen,  that  solid  bodies 
retain  their  form,  nor  that  they  have  a  limited 


THE  VARIOUS  STATES  OF  MATTER          131 

elasticity,  for  M.  Spring  has  made  known  a   case 
where  the  elasticity  of  solids  is  without  any  limit. 

It  was  thought  that  in  the  case  of  a  different 
phenomenon  —  that  of  crystallization  —  we  might 
arrive  at  a  clear  distinction,  because  here  we  should 
be  dealing  with  a  specific  quality  ;  and  that  crystal- 
lized bodies  would  be  the  true  solids,  amorphous 
bodies  being  at  that  time  regarded  as  liquids  viscous 
in  the  extreme. 

But  the  studies  of  a  German  physicist,  Professor 
0.  Lehmann,  seem  to  prove  that  even  this  means  is 
not  infallible.  Professor  Lehmann  has  succeeded, 
in  fact,  in  obtaining  with  certain  organic  compounds 
— oleate  of  potassium,  for  instance — under  certain 
conditions  some  peculiar  states  to  which  he  has 
given  the  name  of  semi-fluid  and  liquid  crystals. 
These  singular  phenomena  can  only  be  observed  and 
studied  by  means  of  a  microscope,  and  the  Carlsruhe 
Professor  had  to  devise  an  ingenious  apparatus 
which  enabled  him  to  bring  the  preparation  at  the 
required  temperature  on  to  the  very  plate  of  the 
microscope. 

It  is  thus  made  evident  that  these  bodies  act  on 
polarized  light  in  the  manner  of  a  crystal.  Those 
that  M.  Lehmann  terms  semi-liquid  still  present 
traces  of  polyhedric  delimitation,  but  with  the  peaks 
and  angles  rounded  by  surface-tension,  while  the 
others  tend  to  a  strictly  spherical  form.  The  optical 
examination  of  the  first-named  bodies  is  very  difficult, 


1 32     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

•x. 

because  appearances  may  be  produced  which  are  due 
to  the  phenomena  of  refraction  and  imitate  those  of 
polarization.  For  the  other  kind,  which  are  often 
as  mobile  as  water,  the  fact  that  they  polarize  light 
is  absolutely  unquestionable. 

Unfortunately,  all  these  liquids  are  turbid,  and 
it  may  be  objected  that  they  are  not  homogeneous. 
This  want  of  homogeneity  may,  according  to  M. 
Quincke,  be  due  to  the  existence  of  particles  sus- 
pended in  a  liquid  in  contact  with  another  liquid 
miscible  with  it  and  enveloping  it  as  might  a  mem- 
brane, and  the  phenomena  of  polarization  would  thus 
be  quite  naturally  explained.1 

M.  Tamman  is  of  opinion  that  it  is  more  a  ques- 
tion of  an  emulsion,  and,  on  this  hypothesis,  the 
action  on  light  would  actually  be  that  which  has 
been  observed.  Various  experimenters  have  en- 
deavoured of  recent  years  to  elucidate  this  question. 
It  cannot  be  considered  absolutely  settled,  but 
these  very  curious  experiments,  pursued  with  great 
patience  and  remarkable  ingenuity,  allow  us  to  think 
that  there  really  exist  certain  intermediary  forms 
between  crystals  and  liquids  in  which  bodies  still 
retain  a  peculiar  structure,  and  consequently  act  on 
light,  but  nevertheless  possess  considerable  plasticity. 

1  Professor  Quincke's  last  hypothesis  is  that  all  liquids  on 
solidifying  pass  through  a  stage  intermediate  between  solid 
and  liquid,  in  which  they  form  what  he  calls  "foam-cells," 
and  assume  a  viscous  structure  resembliug  that  of  jelly.  See 
Proc.  Roy.  Soc.  A.,  23rd  July  1906.— ED. 


THE  VARIOUS  STATES  OF  MATTER         133 

Let  us  note  that  the  question  of  the  continuity  of 
the  liquid  and  solid  states  is  not  quite  the  same  as 
the  question  of  knowing  whether  there  exist  bodies 
intermediate  in  all  respects  between  the  solids  and 
liquids.  These  two  problems  are  often  wrongly  con- 
fused. The  gap  between  the  two  classes  of  bodies 
may  be  filled  by  certain  substances  with  intermediate 
properties,  such  as  pasty  bodies  and  bodies  liquid 
but  still  crystallized,  because  they  have  not  yet  com- 
pletely lost  their  peculiar  structure.  Yet  the  transi- 
tion is  not  necessarily  established  in  a  continuous 
fashion  when  we  are  dealing  with  the  passage  of 
one  and  the  same  determinate  substance  from  the 
liquid  to  the  solid  form.  We  conceive  that  this 
change  may  take  place  by  insensible  degrees  in  the 
case  of  an  amorphous  body.  But  it  seems  hardly 
possible  to  consider  the  case  of  a  crystal,  in  which 
molecular  movements  must  be  essentially  regular, 
as  a  natural  sequence  to  the  case  of  the  liquid 
where  we  are,  on  the  contrary,  in  presence  of  an 
extremely  disordered  state  of  movement. 

M.  Tamman  has  demonstrated  that  amorphous 
solids  may  very  well,  in  fact,  be  regarded  as  super- 
posed liquids  endowed  with  very  great  viscosity. 
But  it  is  no  longer  the  same  thing  when  the  solid 
is  once  in  the  crystallized  state.  There  is  then  a 
solution  of  continuity  of  the  various  properties  of 
the  substance,  and  the  two  phases  may  co-exist. 

We  might  presume   also,  by  analogy  with  what 


134     THE  NEW  PHYSIOS  AND   ITS  EVOLUTION 

happens  with  liquids  and  gases,  that  if  we  followed 
the  curve  of  transformation  of  the  crystalline  into 
the  liquid  phase,  we  might  arrive  at  a  kind  of 
critical  point  at  which  the  discontinuity  of  their 
properties  would  vanish. 

Professor  Poynting,  and  after  him  Professor 
Planck  and  Professor  Ostwald,  supposed  this  to 
be  the  case,  but  more  recently  M.  Tamman  has 
shown  that  such  a  point  does  not  exist,  and  that 
the  region  of  stability  of  the  crystallized  state  is 
limited  on  all  sides.  All  along  the  curve  of  trans- 
formation the  two  states  may  exist  in  equilibrium, 
but  we  may  assert  that  it  is  impossible  to  realize  a 
continuous  series  of  intermediaries  between  these 
two  states.  There  will  always  be  a  more  or  less 
marked  discontinuity  in  some  of  the  properties. 

In  the  course  of  his  researches  M.  Tamman  has 
been  led  to  certain  very  important  observations,  and 
has  met  with  fresh  allotropic  modifications  in  nearly 
all  substances,  which  singularly  complicate  the  ques- 
tion. In  the  case  of  water,  for  instance,  he  finds 
that  ordinary  ice  transforms  itself,  under  a  given 
pressure,  at  the  temperature  of  —  80°  C.  into  another 
crystalline  variety  which  is  denser  than  water. 

The  statics  of  solids  under  high  pressure  is  as  yet, 
therefore,  hardly  drafted,  but  it  seems  to  promise 
results  which  will  not  be  identical  with  those 
obtained  for  the  statics  of  fluids,  though  it  will 
present  at  least  an  equal  interest. 


THE  VARIOUS  STATES  OF   MATTER         135 

§  4.  THE  DEFORMATIONS  OF  SOLIDS 

If  the  mechanical  properties  of  the  bodies  inter- 
mediate between  solids  and  liquids  have  only  lately 
been  the  object  of  systematic  studies,  admittedly 
solid  substances  have  been  studied  for  a  long  time. 
Yet,  notwithstanding  the  abundance  of  researches 
published  on  elasticity  by  theorists  and  experimenters, 
numerous  questions  with  regard  to  them  still  remain 
in  suspense. 

We  only  propose  to  briefly  indicate  here  a  few 
problems  recently  examined,  without  going  into  the 
details  of  questions  which  belong  more  to  the  domain 
of  mechanics  than  to  that  of  pure  physics. 

The  deformations  produced  in  solid  bodies  by 
increasing  efforts  arrange  themselves  in  two  distinct 
periods.  If  the  efforts  are  weak,  the  deformations  pro- 
duced are  also  very  weak  and  disappear  when  the 
effort  ceases.  They  are  then  termed  elastic.  If  the 
efforts  exceed  a  certain  value,  a  part  only  of  these 
deformations  disappear,  and  a  part  are  permanent. 

The  purity  of  the  note  emitted  by  a  sound  has 
been  often  invoked  as  a  proof  of  the  perfect 
isochronism  of  the  oscillation,  and,  consequently, 
as  a  demonstration  a  posteriori  of  the  correctness 
of  the  early  law  of  Hoocke  governing  elastic 
deformations.  This  law  has,  however,  during  some 
years  been  frequently  disputed.  Certain  mechan- 
icians or  physicists  freely  admit  it  to  be  incorrect, 


136    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

especially  as  regards  extremely  weak  deformations. 
According  to  a  theory  in  some  favour,  especially  in 
Germany,  i.e.  the  theory  of  Bach,  the  law  which  con- 
nects the  elastic  deformations  with  the  efforts  would 
be  an  exponential  one.  Recent  experiments  by  Pro- 
fessors Kohlrausch  and  Griineisen,  executed  under 
varied  and  precise  conditions  on  brass,  cast  iron, 
slate,  and  wrought  iron,  do  not  appear  to  confirm 
Bach's  law.  Nothing,  in  point  of  fact,  authorises 
the  rejection  of  the  law  of  Hoocke,  which  presents 
itself  as  the  most  natural  and  most  simple  approxi- 
mation to  reality. 

The  phenomena  of  permanent  deformation  are 
very  complex,  and  it  certainly  seems  that  they 
cannot  be  explained  by  the  older  theories  which 
insisted  that  the  molecules  only  acted  along  the 
straight  line  which  joined  their  centres.  It  becomes 
necessary,  then,  to  construct  more  complete  hypo- 
theses, as  the  MM.  Cosserat  have  done  in  some 
excellent  memoirs,  and  we  may  then  succeed  in  group- 
ing together  the  facts  resulting  from  new  experi- 
ments. Among  the  experiments  of  which  every 
theory  must  take  account  may  be  mentioned  those 
by  which  Colonel  Hartmann  has  placed  in  evidence 
the  importance  of  the  lines  which  are  produced  on 
the  surface  of  metals  when  the  limit  of  elasticity  is 
exceeded. 

It  is  to  questions  of  the  same  order  that  the 
minute  and  patient  researches  of  M.  Bouasse  have 


THE  VARIOUS   STATES  OF   MATTER         137 

been  directed.  This  physicist,  as  ingenious  as  he 
is  profound,  has  pursued  for  several  years  experi- 
ments on  the  most  delicate  points  relating  to  the 
theory  of  elasticity,  and  he  has  succeeded  in  defin- 
ing with  a  precision  not  always  attained  even  in  the 
best  esteemed  works,  the  deformations  to  which  a 
body  must  be  subjected  in  order  to  obtain  compar- 
able experiments.  With  regard  to  the  slight  oscilla- 
tions of  torsion  which  he  has  specially  studied, 
M.  Bouasse  arrives  at  the  conclusion,  in  an  acute 
discussion,  that  we  hardly  know  anything  more  than 
was  proclaimed  a  hundred  years  ago  by  Coulomb. 
We  see,  by  this  example,  that  admirable  as  is  the 
progress  accomplished  in  certain  regions  of  physics, 
there  still  exist  many  over-neglected  regions  which 
remain  in  painful  darkness.  The  skill  shown  by 
M.  Bouasse  authorises  us  to  hope  that,  thanks  to 
his  researches,  a  strong  light  will  some  day  illumine 
these  unknown  corners. 

A  particularly  interesting  chapter  on  elasticity  is 
that  relating  to  the  study  of  crystals;  and  in  the 
last  few  years  it  has  been  the  object  of  remarkable 
researches  on  the  part  of  M.  Voigt.  These  researches 
have  permitted  a  few  controversial  questions  between 
theorists  and  experimenters  to  be  solved:  in  par- 
ticular, M.  Voigt  has  verified  the  consequences  of  the 
calculations,  taking  care  not  to  make,  like  Cauchy 
and  Poisson,  the  hypothesis  of  central  forces  a  mere 
function  of  distance,  and  has  recognized  a  potential 


138    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

which  depends  on  the  relative  orientation  of  the 
molecules.  These  considerations  also  apply  to  quasi- 
isotropic  bodies  which  are,  in  fact,  networks  of 
crystals. 

Certain  occasional  deformations  which  are  pro- 
duced and  disappear  slowly  may  be  considered  as 
intermediate  between  elastic  and  permanent  defor- 
mations. Of  these,  the  thermal  deformation  of 
glass  which  manifests  itself  by  the  displacement  of 
the  zero  of  a  thermometer  is  an  example.  So  also 
the  modifications  which  the  phenomena  of  magnetic 
hysteresis  or  the  variations  of  resistivity  have  just 
demonstrated. 

Many  theorists  have  taken  in  hand  these  difficult 
questions.  M.  Brillouin  endeavours  to  interpret 
these  various  phenomena  by  the  molecular  hypo- 
thesis. The  attempt  may  seem  bold,  since  these 
phenomena  are,  for  the  most  part,  essentially  irre- 
versible, and  seem,  consequently,  not  adaptable  to 
mechanics.  But  M.  Brillouin  makes  a  point 
of  showing  that,  under  certain  conditions,  irre- 
versible phenomena  may  be  created  between  two 
material  points,  the  actions  of  which  depend  solely 
on  their  distance  ;  and  he  furnishes  striking  instances 
which  appear  to  prove  that  a  great  number  of 
irreversible  physical  and  chemical  phenomena  may 
be  ascribed  to  the  existence  of  states  of  unstable 
equilibria. 

M.   Duhem   has   approached   the    problem    from 


THE  VARIOUS  STATES  OF   MATTER         139 

another  side,  and  endeavours  to  bring  it  within  the 
range  of  thermodynamics.  Yet  ordinary  thermo- 
dynamics could  not  account  for  experimentally  realiz- 
able states  of  equilibrium  in  the  phenomena  of 
viscosity  and  friction,  since  this  science  declares  them 
to  be  impossible.  M.  Duhem,  however,  arrives  at  the 
idea  that  the  establishment  of  the  equations  of  thermo- 
dynamics presupposes,  among  other  hypotheses,  one 
which  is  entirely  arbitrary,  namely :  that  when  the 
state  of  the  system  is  given,  external  actions  capable 
of  maintaining  it  in  that  state  are  determined  with- 
out ambiguity,  by  equations  termed  conditions  of 
equilibrium  of  the  system.  If  we  reject  this  hypo- 
thesis, it  will  then  be  allowable  to  introduce  into 
thermodynamics  laws  previously  excluded,  and  it  will 
be  possible  to  construct,  as  M.  Duhem  has  done,  a 
much  more  comprehensive  theory. 

The  ideas  of  M.  Duhem  have  been  illustrated  by 
remarkable  experimental  work.  M.  Marchis,  for 
example,  guided  by  these  ideas,  has  studied  the 
permanent  modifications  produced  in  glass  by  an 
oscillation  of  temperature.  These  modifications, 
which  may  be  called  phenomena  of  the  hysteresis  of 
dilatation,  may  be  followed  in  very  appreciable  fashion 
by  means  of  a  glass  thermometer.  The  general 
results  are  quite  in  accord  with  the  previsions  of 
M.  Duhem.  M.  Lenoble  in  researches  on  the  trac- 
tion of  metallic  wires,  and  M.  Chevalier  in  experi- 
ments on  the  permanent  variations  of  the  electrical 


HO    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

resistance  of  wires  of  an  alloy  of  platinum  and 
silver  when  submitted  to  periodical  variations  of 
temperature,  have  likewise  afforded  verifications  of 
the  theory  propounded  by  M.  Duhem. 

In  this  theory,  the  representative  system  is  con- 
sidered dependent  on  the  temperature  of  one  or 
several  other  variables,  such  as,  for  example,  a 
chemical  variable.  A  similar  idea  has  been  de- 
veloped in  a  very  fine  set  of  memoirs  on  nickel 
steel,  by  M.  Ch.  Ed.  Guillaume.  The  eminent 
physicist,  who,  by  his  earlier  researches,  has 
greatly  contributed  to  the  light  thrown  on 
the  analogous  question  of  the  displacement  of  the 
zero  in  thermometers,  concludes,  from  fresh  re- 
searches, that  the  residual  phenomena  are  due  to 
chemical  variations,  and  that  the  return  to  the 
primary  chemical  state  causes  the  variation  to  dis- 
appear. He  applies  his  ideas  not  only  to  the 
phenomena  presented  by  irreversible  steels,  but  also 
to  very  different  facts;  for  example,  to  phosphor- 
escence, certain  particularities  of  which  may  be 
interpreted  in  an  analogous  manner. 

Nickel  steels  present  the  most  curious  properties, 
and  I  have  already  pointed  out  the  paramount 
importance  of  one  of  them,  hardly  capable  of  per- 
ceptible dilatation,  for  its  application  to  metrology 
and  chronometry.1  Others,  also  discovered  by  M. 
Guillaume  in  the  course  of  studies  conducted 
1  The  metal  known  as  "  invar." — ED. 


THE  VARIOUS  STATES  OF  MATTER         r4i 

with  rare  success  and  remarkable  ingenuity,  may 
render  great  services,  because  it  is  possible  to 
regulate,  so  to  speak,  at  will  their  mechanical  or 
magnetic  properties. 

The  study  of  alloys  in  general  is,  moreover,  one  of 
those  in  which  the  introduction  of  the  methods  of 
physics  has  produced  the  greatest  effects.  By  the 
microscopic  examination  of  a  polished  surface  or  of 
one  indented  by  a  reagent,  by  the  determination  of 
the  electromotive  force  of  elements  of  which  an  alloy 
forms  one  of  the  poles,  and  by  the  measurement  of 
the  resistivities,  the  densities,  and  the  differences 
of  potential  or  contact,  the  most  valuable  indications 
as  to  their  constitution  are  obtained.  M.  Le  Chatelier, 
M.  Charpy,  M.  Dumas,  M.  Osmond,  in  France ;  Sir 
W.  Eoberts  Austen  and  Mr  Stansfield,  in  England, 
have  given  manifold  examples  of  the  fertility  of 
these  methods.  The  question,  moreover,  has  had  a 
new  light  thrown  upon  it  by  the  application  of  the 
principles  of  thermodynamics  and  of  the  phase  rule. 

Alloys  are  generally  known  in  the  two  states 
of  solid  and  liquid.  Fused  alloys  consist  of  one 
or  several  solutions  of  the  component  metals  and  of 
a  certain  number  of  definite  combinations.  Their 
composition  may  thus  be  very  complex :  but 
Gibbs'  rule  gives  us  at  once  important  information 
on  the  point,  since  it  indicates  that  there  cannot 
exist,  in  general,  more  than  two  distinct  solutions 
in  an  alloy  of  two  metals. 


142     THE   NEW  PHYSICS  AND   ITS   EVOLUTION 

Solid  alloys  may  be  classed  like  liquid  ones.  Two 
metals  or  more  dissolve  one  into  the  other,  and  form 
a  solid  solution  quite  analogous  to  the  liquid  solution. 
But  the  study  of  these  solid  solutions  is  rendered 
singularly  difficult  by  the  fact  that  the  equili- 
brium so  rapidly  reached  in  the  case  of  liquids  in 
this  case  takes  days  and,  in  certain  cases,  perhaps 
even  centuries  to  become  established. 


CHAPTEE  V 

SOLUTIONS  AND  ELECTROLYTIC 
DISSOCIATION 

§  1.  SOLUTION 

VAPORIZATION  and  fusion  are  not  the  only  means  by 
which  the  physical  state  of  a  body  may  be  changed 
without  modifying  its  chemical  constitution.  From 
the  most  remote  periods  solution  has  also  been  known 
and  studied,  but  only  in  the  last  twenty  years  have 
we  obtained  other  than  empirical  information  regard- 
ing this  phenomenon. 

It  is  natural  to  employ  here  also  the  methods 
which  have  allowed  us  to  penetrate  into  the  know- 
ledge of  other  transformations.  The  problem  of 
solution  may  be  approached  by  way  of  thermo- 
dynamics and  of  the  hypotheses  of  kinetics. 

As  long  ago  as  1858,  Kirchhoff,  by  attributing  to 
saline  solutions — that  is  to  say,  to  mixtures  of  water 
and  a  non-volatile  liquid  like  sulphuric  acid — the 
properties  of  internal  energy,  discovered  a  relation 
between  the  quantity  of  heat  given  out  on  the 


143 


144    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

addition  of  a  certain  quantity  of  water  to  a  solu- 
tion and  the  variations  to  which  condensation  and 
temperature  subject  the  vapour-tension  of  the  solu- 
tion. He  calculated  for  this  purpose  the  variations 
of  energy  which  are  produced  when  passing  from 
one  state  to  another  by  two  different  series  of 
transformations;  and,  by  comparing  the  two  ex- 
pressions thus  obtained,  he  established  a  relation 
between  the  various  elements  of  the  phenomenon. 
But,  for  a  long  time  afterwards,  the  question  made 
little  progress,  because  there  seemed  to  be  hardly 
any  means  of  introducing  into  this  study  the  second 
principle  of  thermodynamics.1  It  was  the  memoir 
of  Gibbs  which  at  last  opened  out  this  rich  domain 
and  enabled  it  to  be  rationally  exploited.  As  early 
as  1886,  M.  Duhem  showed  that  the  theory  of  the 
thermodynamic  potential  furnished  precise  infor- 
mation on  solutions  or  liquid  mixtures.  He  thus 
discovered  over  again  the  famous  law  on  the  lowering 

1  The  "  second  principle  "  referred  to  has  been  thus  enunci- 
ated :  "  In  every  engine  that  produces  work  there  is  a  fall  of 
temperature,  and  the  maximum  output  of  a  perfect  engine — 
i.e.  the  ratio  between  the  heat  consumed  in  work  and  the 
heat  supplied — depends  only  on  the  extreme  temperatures 
between  which  the  fluid  is  evolved." — Demanet,  Notes  de 
Physique  Experimentale,  Lou  vain,  1905,  fasc.  2,  p.  147. 
Clausius  put  it  in  a  negative  form,  as  thus  :  No  engine  can 
of  itself,  without  the  aid  of  external  agency,  transfer  heat 
from  a  body  at  low  temperature  to  a  body  at  a  high 
temperature.  Cf.  Ganot's  Physics,  17th  English  edition, 
§  508.— ED. 


SOLUTIONS  &  ELECTROLYTIC    DISSOCIATION    145 

of  the  congelation  temperature  of  solvents  which  had 
just  been  established  by  M.  Eaoult  after  a  long  series 
of  now  classic  researches. 

In  the  minds  of  many  persons,  however,  grave 
doubts  persisted.  Solution  appeared  to  be  an 
essentially  irreversible  phenomenon.  It 'was  there- 
fore, in  all  strictness,  impossible  to  calculate  the 
entropy  of  a  solution,  and  consequently  to  be 
certain  of  the  value  of  the  thermodynamic  potential. 
The  objection  would  be  serious  even  to-day,  and,  in 
calculations,  what  is  called  the  paradox  of  Gibbs 
would  be  an  obstacle. 

We  should  not  hesitate,  however,  to  apply  the 
Phase  Law  to  solutions,  and  this  law  already  gives 
us  the  key  to  a  certain  number  of  facts.  It  puts 
in  evidence,  for  example,  the  part  played  by  the 
eutectic  point — that  is  to  say,  the  point  at  which 
(to  keep  to  the  simple  case  in  which  we  have  to  do 
with  two  bodies  only,  the  solvent  and  the  solute)  the 
solution  is  in  equilibrium  at  once  with  the  two 
possible  solids,  the  dissolved  body  and  the  solvent 
solidified.  The  knowledge  of  this  point  explains  the 
properties  of  refrigerating  mixtures,  and  it  is  also 
one  of  the  most  useful  for  the  theory  of  alloys.  The 
scruples  of  physicists  ought  to  have  been  removed  on 
the  memorable  occasion  when  Professor  Van  t'  Hoff 
demonstrated  that  solution  can  operate  reversibly 
by  reason  of  the  phenomena  of  osmosis.  But  the 
experiment  can  only  succeed  in  very  rare  cases ; 

10 


i46    THE  NEW  PHYSICS   AND  ITS  EVOLUTION 

and,  on  the  other  hand,  Professor  Van  t'  Hoff  was 
naturally  led  to  another  very  bold  conception.  He 
regarded  the  molecule  of  the  dissolved  body  as  a 
gaseous  one,  and  assimilated  solution,  not  as  had 
hitherto  been  the  rule,  to  fusion,  but  to  a  kind  of 
vaporization.  Naturally  his  ideas  were  not  immedi- 
ately accepted  by  the  scholars  most  closely  identified 
with  the  classic  tradition.  It  may  perhaps  not  be 
without  use  to  examine  here  the  principles  of 
Professor  Van  t'  HofFs  theory. 

§  2.  OSMOSIS 

Osmosis,  or  diffusion  through  a  septum,  is  a 
phenomenon  which  has  been  known  for  some  time. 
The  discovery  of  it  is  attributed  to  the  Abbe  Nollet, 
who  is  supposed  to  have  observed  it  in  1748, 
during  some  "researches  on  liquids  in  ebullition.'' 
A  classic  experiment  by  Dutrochet,  effected  about 
1830,  makes  this  phenomenon  clear.  Into  pure 
water  is  plunged  the  lower  part  of  a  vertical  tube 
containing  pure  alcohol,  open  at  the  top  and  closed  at 
the  bottom  by  a  membrane,  such  as  a  pig's  bladder, 
without  any  visible  perforation.  In  a  very  short 
time  it  will  be  found,  by  means  of  an  areometer  for 
instance,  that  the  water  outside  contains  alcohol, 
while  the  alcohol  of  the  tube,  pure  at  first,  is 
now  diluted.  Two  currents  have  therefore  passed 
through  the  membrane,  one  of  water  from  the  out- 
side to  the  inside,  and  one  of  alcohol  in  the  converse 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION     147 

direction.  It  is  also  noted  that  a  difference  in  the 
levels  has  occurred,  and  that  the  liquid  in  the  tube 
now  rises  to  a  considerable  height.  It  must  there- 
fore be  admitted  that  the  flow  of  the  water  has  been 
more  rapid  than  that  of  the  alcohol.  At  the  com- 
mencement, the  water  must  have  penetrated  into 
the  tube  much  more  rapidly  than  the  alcohol  left  it. 
Hence  the  difference  in  the  levels,  and,  consequently, 
a  difference  of  pressure  on  the  two  faces  of  the 
membrane.  This  difference  goes  on  increasing, 
reaches  a  maximum,  then  diminishes,  and  vanishes 
when  the  diffusion  is  complete,  final  equilibrium 
being  then  attained. 

The  phenomenon  is  evidently  connected  with 
diffusion.  If  water  is  very  carefully  poured  on  to 
alcohol,  the  two  layers,  separate  at  first,  mingle  by 
degrees  till  a  homogeneous  substance  is  obtained. 
The  bladder  seems  not  to  have  prevented  this  diffusion 
from  taking  place,  but  it  seems  to  have  shown  itself 
more  permeable  to  water  than  to  alcohol.  May  it  not 
therefore  be  supposed  that  there  must  exist  dividing 
walls  in  which  this  difference  of  permeability 
becomes  greater  and  greater,  which  would  be  per- 
meable to  the  solvent  and  absolutely  impermeable 
to  the  solute  ?  If  this  be  so,  the  phenomena  of 
these  semi-permeable  walls,  as  they  are  termed,  can 
be  observed  in  particularly  simple  conditions. 

The  answer  to  this  question  has  been  furnished 
by  biologists,  at  which  we  cannot  be  surprised.  The 


148    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

phenomena  of  osmosis  are  naturally  of  the  first 
importance  in  the  action  of  organisms,  and  for  a 
long  time  have  attracted  the  attention  of  naturalists. 
De  Vries  imagined  that  the  contractions  noticed  in 
the  protoplasm  of  cells  placed  in  saline  solutions 
were  due  to  a  phenomenon  of  osmosis,  and,  upon 
examining  more  closely  certain  peculiarities  of  cell 
life,  various  scholars  have  demonstrated  that  living 
cells  are  enclosed  in  membranes  permeable  to  certain 
substances  and  entirely  impermeable  to  others.  It 
was  interesting  to  try  to  reproduce  artificially  semi- 
permeable  walls  analogous  to  those  thus  met  with 
in  nature l ;  and  Traube  and  Pf effer  seem  to  have 
succeeded  in  one  particular  case.  Traube  has  pointed 
out  that  the  very  delicate  membrane  of  ferrocyanide 
of  potassium  which  is  obtained  with  some  difficulty 
by  exposing  it  to  the  reaction  of  sulphate  of  copper, 
is  permeable  to  water,  but  will  not  permit  the 
passage  of  the  majority  of  salts.  Pfeffer,  by  pro- 
ducing these  walls  in  the  interstices  of  a  porous 
porcelain,  has  succeeded  in  giving  them  sufficient 
rigidity  to  allow  measurements  to  be  made.  It  must 
be  allowed  that,  unfortunately,  no  physicist  or 
chemist  has  been  as  lucky  as  these  two  botanists ; 
and  the  attempts  to  reproduce  semi-permeable  walls 
completely  answering  to  the  definition,  have  never 
given  but  mediocre  results.  If,  however,  the  experi- 
mental difficulty  has  not  been  overcome  in  an 
1  See  next  note.— ED. 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    149 

entirely  satisfactory  manner,  it  at  least  appears 
very  probable  that  such  walls  may  nevertheless 
exist.1 

Nevertheless,  in  the  case  of  gases,  there  exists 
an  excellent  example  of  a  semi-permeable  wall,  and 
a  partition  of  platinum  brought  to  a  higher  than  red 
heat  is,  as  shown  by  M.  Villard  in  some  ingenious 
experiments,  completely  impermeable  to  air,  and  very 
permeable,  on  the  contrary,  to  hydrogen.  It  can 
also  be  experimentally  demonstrated  that  on  taking 
two  recipients  separated  by  such  a  partition,  and  both 
containing  nitrogen  mixed  with  varying  proportions 
of  hydrogen,  the  last-named  gas  will  pass  through 
the  partition  in  such  a  way  that  the  concentration — 
that  is  to  say;  the  mass  of  gas  per  unit  of  volume — 
will  become  the  same  on  both  sides.  Only  then  will 
equilibrium  be  established ;  and,  at  that  moment,  an 
excess  of  pressure  will  naturally  be  produced  in  that 
recipient  which,  at  the  commencement,  contained  the 
gas  with  the  smallest  quantity  of  hydrogen. 

This  experiment  enables  us  to  anticipate  what  will 
happen  in  a  liquid  medium  with  semi-permeable 
partitions.  Between  two  recipients,  one  containing 
pure  water,  the  other,  say,  water  with  sugar  in 

1  M.  Stephane  Leduc,  Professor  of  Biology  of  Nantes,  has 
made  many  experiments  in  this  connection,  and  the  artificial 
cells  exhibited  by  him  to  the  Association  frangaise  pour 
1'avancement  des  Sciences,  at  their  meeting  at  Grenoble 
in  1904  and  reproduced  in  their  "Actes,"  are  particularly 
noteworthy.  — ED. 


i5o    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

solution,  separated  by  one  of  these  partitions,  there 
will  be  produced  merely  a  movement  of  the  pure 
towards  the  sugared  water,  and  following  this,  an 
increase  of  pressure  on  the  side  of  the  last.  But 
this  increase  will  not  be  without  limits.  At  a 
certain  moment  the  pressure  will  cease  to  increase 
and  will  remain  at  a  fixed  value  which  now  has  a 
given  direction.  This  is  the  osmotic  pressure. 

Pfeffer  demonstrated  that,  for  the  same  substance, 
the  osmotic  pressure  is  proportional  to  the  con- 
centration, and  consequently  in  inverse  ratio  to  the 
volume  occupied  by  a  similar  mass  of  the  solute. 
He  gave  figures  from  which  it  was  easy,  as  Professor 
Van  t'  Hoff  found,  to  draw  the  conclusion  that,  in 
a  constant  volume,  the  osmotic  pressure  is  pro- 
portional to  the  absolute  temperature.  De  Vries, 
moreover,  by  his  remarks  on  living  cells,  extended 
the  results  which  Pfeffer  had  applied  to  one  case 
only — that  is,  to  the  one  that  he  had  been  able  to 
examine  experimentally. 

Such  are  the  essential  facts  of  osmosis.  We  may 
seek  to  interpret  them  and  to  thoroughly  examine 
the  mechanism  of  the  phenomenon ;  but  it  must  be 
acknowledged  that  as  regards  this  point,  physicists 
are  not  entirely  in  accord.  In  the  opinion  of 
Professor  Nernst,  the  permeability  of  semi-permeable 
membranes  is  simply  due  to  differences  of  solubility 
in  one  of  the  substances  of  the  membrane  itself. 
Other  physicists  think  it  attributable,  either  to 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    151 

the  difference  in  the  dimensions  of  the  molecules, 
of  which  some  might  pass  through  the  pores  of  the 
membrane  and  others  be  stopped  by  their  relative 
size,  or  to  these  molecules'  greater  or  less  mobility. 
For  others,  again,  it  is  the  capillary  phenomena 
which  here  act  a  preponderating  part. 

This  last  idea  is  already  an  old  one  :  Jager,  More, 
and  Professor  Traube  have  all  endeavoured  to  show 
that  the  direction  and  speed  of  osmosis  are  deter- 
mined by  differences  in  the  surface-tensions;  and 
recent  experiments,  especially  those  of  Batelli,  seem 
to  prove  that  osmosis  establishes  itself  in  the  way 
which  best  equalizes  the  surface-tensions  of  the 
liquids  on  both  sides  of  the  partition.  Solutions 
possessing  the  same  surface-tension,  though  not 
in  molecular  equilibrium,  would  thus  be  always  in 
osmotic  equilibrium.  We  must  not  conceal  from 
ourselves  that  this  result  would  be  in  contradiction 
with  the  kinetic  theory. 

§  3.  APPLICATION  TO  THE  THEORY  OF  SOLUTION 

If  there  really  exist  partitions  permeable  to  one 
body  and  impermeable  to  another,  it  may  be  imagined 
that  the  homogeneous  mixture  of  these  two  bodies 
might  be  effected  in  the  converse  way.  It  can  be 
easily  conceived,  in  fact,  that  by  the  aid  of  osmotic 
pressure  it  would  be  possible,  for  example,  to  dilute 
or  concentrate  a  solution  by  driving  through  the 
partition  in  one  direction  or  another  a  certain 


152    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

quantity  of  the  solvent  by  means  of  a  pressure  kept 
equal  to  the  osmotic  pressure.  This  is  the  important 
fact  which  Professor  Van  t'  Hoff  perceived.  The 
existence  of  such  a  wall  in  all  possible  cases 
evidently  remains  only  a  very  legitimate  hypothesis, 
— a  fact  which  ought  not  to  be  concealed. 
'  Belying  solely  on  this  postulate,  Professor  Van  t' 
Hoff  easily  established,  by  the  most  correct  method, 
certain  properties  of  the  solutions  of  gases  in  a  volatile 
liquid,  or  of  non-volatile  bodies  in  a  volatile  liquid. 
To  state  precisely  the  other  relations,  we  must  admit, 
in  addition,  the  experimental  laws  discovered  by 
Pfeffer.  But  without  any  hypothesis  it  becomes 
possible  to  demonstrate  the  laws  of  Kaoult  on  the 
lowering  of  the  vapour- tension  and  of  the  freezing 
point  of  solutions,  and  also  the  ratio  which  connects 
the  heat  of  fusion  with  this  decrease. 

These  considerable  results  can  evidently  be  invoked 
as  a  posteriori  proofs  of  the  exactitude  of  the  experi- 
mental laws  of  osmosis.  They  are  not,  however,  the 
only  ones  that  Professor  Van  t'  Hoff  has  obtained  by 
the  same  method.  This  illustrious  scholar  was  thus 
able  to  find  anew  Guldberg  and  Waage's  law  on 
chemical  equilibrium  at  a  constant  temperature,  and 
to  show  how  the  position  of  the  equilibrium  changes 
when  the  temperature  happens  to  change. 

If  now  we  state,  in  conformity  with  the  laws 
of  Pfeffer,  that  the  product  of  the  osmotic  pressure 
by  the  volume  of  the  solution  is  equal  to  the  absolute 


SOLUTIONS  &  ELECTROLYTIC   DISSOCIATION    153 

temperature  multiplied  by  a  coefficient,  and  then 
look  for  the  numerical  figure  of  this  latter  in  a 
solution  of  sugar,  for  instance,  we  find  that  this 
value  is  the  same  as  that  of  the  analogous  coefficient 
of  the  characteristic  equation  of  a  perfect  gas. 
There  is  in  this  a  coincidence  which  has  also  been 
utilized  in  the  preceding  thermodynamic  calcula- 
tions. It  may  be  purely  fortuitous,  but  we  can 
hardly  refrain  from  finding  in  it  a  physical  meaning. 
Professor  Van  t'  Hoff  has  considered  this  coinci- 
dence a  demonstration  that  there  exists  a  strong 
analogy  between  a  body  in  solution  and  a  gas  ;  as  a 
matter  of  fact,  it  may  seem  that,  in  a  solution,  the 
distance  between  the  molecules  becomes  comparable  to 
the  molecular  distances  met  with  in  gases,  and  that 
the  molecule  acquires  the  same  degree  of  liberty  and 
the  same  simplicity  in  both  phenomena.  In  that  case 
it  seems  probable  that  solutions  will  be  subject  to 
laws  independent  of  the  chemical  nature  of  the 
dissolved  molecule  and  comparable  to  the  laws 
governing  gases,  while  if  we  adopt  the  kinetic 
image  for  the  gas,  we  shall  be  led  to  represent  to 
ourselves  in  a  similar  way  the  phenomena  which 
manifest  themselves  in  a  solution.  Osmotic  pres- 
sure will  then  appear  to  be  due  to  the  shock  of 
the  dissolved  molecules  against  the  membrane.  It 
will  come  from  one  side  of  this  partition  to  superpose 
itself  on  the  hydrostatic  pressure,  which  latter  must 
have  the  same  value  on  both  sides. 


154    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

The  analogy  with  a  perfect  gas  naturally  becomes 
much  greater  as  the  solution  becomes  more  diluted. 
It  then  imitates  gas  in  some  other  properties ;  the 
internal  work  of  the  variation  of  volume  is  nil,  and 
the  specific  heat  is  only  a  function  of  the  temperature. 
A  solution  which  is  diluted  by  a  reversible  method 
is  cooled  like  a  gas  which  expands  adiabatically.1 

It  must,  however,  be  acknowledged  that,  in  other 
points,  the  analogy  is  much  less  perfect.  The 
opinion  which  sees  in  solution  a  phenomenon 
resembling  fusion,  and  which  has  left  an  indelible 
trace  in  everyday  language  (we  shall  always  say: 
to  melt  sugar  in  water)  is  certainly  not  without 
foundation.  Certain  of  the  reasons  which  might 
be  invoked  to  uphold  this  opinion  are  too  evident 
to  be  repeated  here,  though  others  more  recondite 
might  be  quoted.  The  fact  that  the  internal  energy 
generally  becomes  independent  of  the  concentration 
when  the  dilution  reaches  even  a  moderately  high 
value  is  rather  in  favour  of  the  hypothesis  of  fusion. 

We  must  not  forget,  however,  the  continuity  of 
the  liquid  and  gaseous  states  ;  and  we  may  consider 
it  in  an  absolute  way  a  question  devoid  of  sense  to 
ask  whether  in  a  solution  the  solute  is  in  the  liquid  or 
the  gaseous  state.  It  is  in  the  fluid  state,  and  perhaps 
in  conditions  opposed  to  those  of  a  body  in  the 
state  of  a  perfect  gas.  It  is  known,  of  course,  that 
in  this  case  the  manometrical  pressure  must  be 

1  That  is,  without  receiving  or  emitting  any  heat. — ED. 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    155 

regarded  as  very  great  in  relation  to  the  internal 
pressure  which,  in  the  characteristic  equation,  is 
added  to  the  other.  May  it  not  seem  possible 
that  in  the  solution  it  is,  on  the  contrary,  the 
internal  pressure  which  is  dominant,  the  manometric 
pressure  becoming  of  no  account  ?  The  coincidence 
of  the  formulas  would  thus  be  verified,  for  all  the 
characteristic  equations  are  symmetrical  with  regard 
to  these  two  pressures.  From  this  point  of  view 
the  osmotic  pressure  would  be  considered  as  the 
result  of  an  attraction  between  the  solvent  and  the 
solute ;  and  it  would  represent  the  difference  between 
the  internal  pressures  of  the  solution  and  of  the 
pure  solvent.  These  hypotheses  are  highly  interest- 
ing, and  very  suggestive ;  but  from  the  way  in 
which  the  facts  have  been  set  forth,  it  will  appear, 
no  doubt,  that  there  is  no  obligation  to  admit  them  in 
order  to  believe  in  the  legitimacy  of  the  application 
of  thermodynamics  to  the  phenomena  of  solution. 

§  4.  ELECTROLYTIC  DISSOCIATION 

From  the  outset  Professor  Van  t'  Hoff  was  brought 
to  acknowledge  that  a  great  number  of  solutions 
formed  very  notable  exceptions  which  were  very 
irregular  in  appearance.  The  analogy  with  gases  did 
not  seem  to  be  maintained,  for  the  osmotic  pressure 
had  a  very  different  value  from  that  indicated  by 
the  theory.  Everything,  however,  came  right  if  one 
multiplied  by  a  factor,  determined  according  to  each 


156    THE   NEW  PHYSICS  AND   ITS   EVOLUTION 

case,  but  greater  than  unity,  the  constant  of  the 
characteristic  formula.  Similar  divergences  were 
manifested  in  the  delays  observed  in  congelation, 
and  disappeared  when  subjected  to  an  analogous 
correction. 

Thus  the  freezing-point  of  a  normal  solution, 
containing  a  molecule  gramme  (that  is,  the  number 
of  grammes  equal  to  the  figure  representing  the 
molecular  mass)  of  alcohol  or  sugar  in  water, 
falls  1'85°  C.  If  the  laws  of  solution  were 
identically  the  same  for  a  solution  of  sea-salt, 
the  same  depression  should  be  noticed  in  a  saline 
solution  also  containing  1  molecule  per  litre.  In 
fact,  the  fall  reaches  3  "26°,  and  the  solution  behaves 
as  if  it  contained,  not  1,  but  1'75  normal  molecules 
per  litre.  The  consideration  of  the  osmotic  pressures 
would  lead  to  similar  observations,  but  we  know 
that  the  experiment  would  be  more  difficult  and 
less  precise. 

We  may  wonder  whether  anything  really 
analogous  to  this  can  be  met  with  in  the  case  of  a 
gas,  and  we  are  thus  led  to  consider  the  phenomena 
of  dissociation.1  If  we  heat  a  body  which,  in  a 

1  Dissociation  must  be  distinguished  from  decomposition, 
which  is  what  occurs  when  the  whole  of  a  particle,  (compound, 
molecule,  atom,  etc.)  breaks  up  into  its  component  parts.  In 
dissociation  the  breaking  up  is  only  partial,  and  the  resultant 
consists  of  a  mixture  of  decomposed  and  undecomposed  parts. 
See  Ganot's  Physics,  17th  English  edition,  §  395,  for  examples. 
—ED. 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    157 

gaseous  state,  is  capable  of  dissociation — hydriodic 
acid,  for  example — at  a  given  temperature,  an 
equilibrium  is  established  between  three  gaseous 
bodies,  the  acid,  the  iodine,  and  the  hydrogen.  The 
total  mass  will  follow  with  fair  closeness  Mariotte's 
law,  but  the  characteristic  constant  will  no  longer  be 
the  same  as  in  the  case  of  a  non-dissociated  gas. 
We  here  no  longer  have  to  do  with  a  single 
molecule,  since  each  molecule  is  in  part  dissociated. 

The  comparison  of  the  two  cases^  leads  to  the 
employment  of  a  new  image  for  representing  the 
phenomenon  which  has  been  produced  throughout 
the  saline  solution.  We  have  introduced  a  single 
molecule  of  salt,  and  everything  occurs  as  if  there 
were  175  molecules.  May  it  not  really  be  said  that 
the  number  is  1/75,  because  the  sea-salt  is  partly 
dissociated,  and  a  molecule  has  become  transformed 
into  0*75  molecule  of  sodium,  O75  of  chlorium,  and 
0-25  of  salt? 

This  is  a  way  of  speaking  which  seems,  at 
first  sight,  strangely  contradicted  by  experiment. 
Professor  Van  t'  Hoff,  like  other  chemists,  would 
certainly  have  rejected — in  fact,  he  did  so  at  first- 
such  a  conception,  if,  about  the  same  time,  an 
illustrious  Swedish  scholar,  M.  Arrhenius,  had  not 
been  brought  to  the  same  idea  by  another  road,  and, 
had  not  by  stating  it  precisely  and  modifying  it, 
presented  it  in  an  acceptable  form. 

A  brief  examination  will  easily  show  that  all  the 


158    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

substances  which  are  exceptions  to  the  laws  of  Van 
t'  Hoff  are  precisely  those  which  are  capable  of 
conducting  electricity  when  undergoing  decomposi- 
tion— that  is  to  say,  are  electrolytes.  The  coincidence 
is  absolute,  and  cannot  be  simply  due  to  chance. 

Now,  the  phenomena  of  electrolysis  have,  for  a 
long  time,  forced  upon  us  an  almost  necessary  image. 
The  saline  molecule  is  always  decomposed,  as  we 
know,  in  the  primary  phenomenon  of  electrolysis 
into  two  elements  which  Faraday  termed  ions. 
Secondary  reactions,  no  doubt,  often  come  to 
complicate  the  question,  but  these  are  chemical  re- 
actions belonging  to  the  general  order  of  things,  and 
have  nothing  to  do  with  the  electric  action  working 
on  the  solution.  The  simple  phenomenon  is  always 
the  same — decomposition  into  two  ions,  followed  by 
the  appearance  of  one  of  these  ions  at  the  positive 
and  of  the  other  at  the  negative  electrode.  But  as 
the  very  slightest  expenditure  of  energy  is  sufficient 
to  produce  the  commencement  of  electrolysis,  it  is 
necessary  to  suppose  that  these  two  ions  are  not 
united  by  any  force.  Thus  the  two  ions  are,  in  a 
way,  dissociated.  Clausius,  who  was  the  first  to 
represent  the  phenomena  by  this  symbol,  supposed, 
in  order  not  to  shock  the  feelings  of  chemists  too 
much,  that  this  dissociation  only  affected  an  infini- 
tesimal fraction  of  the  total  number  of  the  molecules 
of  the  salt,  and  thereby  escaped  all  check. 

This     concession     was     unfortunate,     and      the 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    159 

hypothesis  thus  lost  the  greater  part  of  its  use- 
fulness. M.  Arrhenius  was  bolder,  and  frankly 
recognized  that  dissociation  occurs  at  once  in  the 
case  of  a  great  number  of  molecules,  and  tends 
to  increase  more  and  more  as  the  solution  becomes 
more  dilute.  It  follows  the  comparison  with  a  gas 
which,  while  partially  dissociated  in  an  enclosed 
space,  becomes  wholly  so  in  an  infinite  one. 

M.  Arrhenius  was  led  to  adopt  this  hypothesis 
by  the  examination  of  experimental  results  relating 
to  the  conductivity  of  electrolytes.  In  order  to 
interpret  certain  facts,  it  has  to  be  recognized  that  a 
part  only  of  the  molecules  in  a  saline  solution  can 
be  considered  as  conductors  of  electricity,  and  that 
by  adding  water  the  number  of  molecular  conductors 
is  increased.  This  increase,  too,  though  rapid  at  first, 
soon  becomes  slower,  and  approaches  a  certain  limit 
which  an  infinite  dilution  would  enable  it  to  attain. 
If  the  conducting  molecules  are  the  dissociated 
molecules,  then  the  dissociation  (so  long  as  it  is  a 
question  of  strong  acids  and  salts)  tends  to  become 
complete  in  the  case  of  an  unlimited  dilution. 

The  opposition  of  a  large  number  of  chemists  and 
physicists  to  the  ideas  of  M.  Arrhenius  was  at  first 
very  fierce.  It  must  be  noted  with  regret  that,  in 
France  particularly,  recourse  was  had  to  an  arm 
which  scholars  often  wield  rather  clumsily.  They 
joked  about  these  free  ions  in  solution,  and  they 
asked  to  see  this  chlorine  and  this  sodium  which 


160     THE  NEW  PHYSIOS  AND  ITS  EVOLUTION 

swam  about  the  water  in  a  state  of  liberty.  But 
in  science,  as  elsewhere,  irony  is  not  argument,  and 
it  soon  had  to  be  acknowledged  that  the  hypothesis 
of  M.  Arrhenius  showed  itself  singularly  fertile 
and  had  to  be  regarded,  at  all  events,  as  a  very 
expressive  image,  if  not,  indeed,  entirely  in  con- 
formity with  reality. 

It  would  certainly  be  contrary  to  all  experience, 
and  even  to  common  sense  itself,  to  suppose  that 
in  dissolved  chloride  of  sodium  there  is  really  free 
sodium,  if  we  suppose  these  atoms  of  sodium  to 
be  absolutely  identical  with  ordinary  atoms.  But 
there  is  a  great  difference.  In  the  one  case  the 
atoms  are  electrified,  and  carry  a  relatively  consider- 
able positive  charge,  inseparable  from  their  state  as 
ions,  while  in  the  other  they  are  in  the  neutral  state. 
We  may  suppose  that  the  presence  of  this  charge 
brings  about  modifications  as  extensive  as  one  pleases 
in  the  chemical  properties  of  the  atom.  Thus  the 
hypothesis  will  be  removed  from  all  discussion  of 
a  chemical  order,  since  it  will  have  been  made  plastic 
enough  beforehand  to  adapt  itself  to  all  the  known 
facts ;  and  if  we  object  that  sodium  cannot  subsist 
in  water  because  it  instantaneously  decomposes  the 
latter,  the  answer  is  simply  that  the  sodium  ion  does 
not  decompose  water  as  does  ordinary  sodium. 

Still,  other  objections  might  be  raised  which  could 
not  be  so  easily  refuted.  One,  to  which  chemists 
not  unreasonably  attached  great  importance,  was 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    161 

this: — If  a  certain  quantity  of  chloride  of  sodium 
is  dissociated  into  chlorine  and  sodium,  it  should  be 
possible,  by  diffusion,  for  example,  which  brings  out 
plainly  the  phenomena  of  dissociation  in  gases,  to 
extract  from  the  solution  a  part  either  of  the 
chlorine  or  of  the  sodium,  while  the  corresponding 
part  of  the  other  compound  \vould  remain.  This 
result  would  be  in  flagrant  contradiction  with  the 
fact  that,  everywhere  and  always,  a  solution  of  salt 
contains  strictly  the  same  proportions  of  its  com- 
ponent elements. 

M.  Arrhenius  answers  to  this  that  the  electrical 
forces  in  ordinary  conditions  prevent  separation  by 
diffusion  or  by  any  other  process.  Professor  Nernst 
goes  further,  and  has  shown  that  the  concentration 
currents  which  aret produced  when  two  electrodes  of 
the  same  substance  are  plunged  into  two  unequally 
concentrated  solutions  may  be  interpreted  by  the 
hypothesis  that,  in  these  particular  conditions,  the 
diffusion  does  bring  about  a  separation  of  the  ions. 
Thus  the  argument  is  turned  round,  and  the  proof 
supposed  to  be  given  of  the  incorrectness  of  the 
theory  becomes  a  further  reason  in  its  favour. 

It  is  possible,  no  doubt,  to  adduce  a  few  other 
experiments  which  are  not  very  favourable  to  M. 
Arrhenius's  point  of  view,  but  they  are  isolated 
cases  ;  and,  on  the  whole,  his  theory  has  enabled  many 
isolated  facts,  till  then  scattered,  to  be  co-ordinated, 
and  has  allowed  very  varied  phenomena  to  be  linked 

II 


1 62    THE   NEW  PHYSICS  AND   ITS   EVOLUTION 

together.  It  has  also  suggested — and,  moreover,  still 
daily  suggests — researches  of  the  highest  order. 

In  the  first  place,  the  theory  of  Arrhenius  explains 
electrolysis  very  simply.  The  ions  which,  so  to  speak, 
wander  about  haphazard,  and  are  uniformly  distri- 
buted throughout  the  liquid,  steer  a  regular  course  as 
soon  as  we  dip  in  the  trough  containing  the  electrolyte 
the  two  electrodes  connected  with  the  poles  of  the 
dynamo  or  generator  of  electricity.  Then  the  charged 
positive  ions  travel  in  the  direction  of  the  electro- 
motive force  and  the  negative  ions  in  the  opposite 
direction.  On  reaching  the  electrodes  they  yield  up 
to  them  the  charges  they  carry,  and  thus  pass  from 
the  state  of  ion  into  that  of  ordinary  atom.  More- 
over, for  the  solution  to  remain  in  equilibrium, 
the  vanished  ions  must  be  immediately  replaced  by 
others,  and  thus  the  ^ state  of  ionisation  of  the 
electrolyte  remains  constant  and  its  conductivity 
persists. 

All  the  peculiarities  of  electrolysis  are  capable 
of  interpretation  :  the  phenomena  of  the  transport 
of  ions,  the  fine  experiments  of  M.  Bouty,  those  of 
Professor  Kohlrausch  and  of  Professor  Ostwald  on 
various  points  in  electrolytic  conduction,  all  support 
the  theory.  The  verifications  of  it  can  even  be 
quantitative,  and  we  can  foresee  numerical  relations 
between  conductivity  and  other  phenomena.  The 
measurement  of  the  conductivity  permits  the  number 
of  molecules  dissociated  in  a  given  solution  to  be 


SOLUTIONS  &   ELECTROLYTIC  DISSOCIATION    163 

calculated,  and  the  number  is  thus  found  to  be 
precisely  the  same  as  that  arrived  at  if  it  is  wished 
to  remove  the  disagreement  between  reality  and  the 
anticipations  which  result  from  the  theory  of  Pro- 
fessor Van  t'  Hoff.  The  laws  of  cryoscopy,  of  tono- 
metry,  and  of  osmosis  thus  again  become  strict,  and 
no  exception  to  them  remains. 

If  the  dissociation  of  salts  is  a  reality  and  is 
complete  in  a  dilute  solution,  any  of  the  properties 
of  a  saline  solution  whatever  should  be  represented 
numerically  as  the  sum  of  three  values,  of  which  one 
concerns  the  positive  ion,  a  second  the  negative 
ion,  and  the  third  the  solvent.  The  properties 
of  the  solutions  would  then  be  what  are  called 
additive  properties.  Numerous  verifications  may  be 
attempted  by  very  different  roads.  They  generally 
succeed  very  well;  and  whether  we  measure  the 
electric  conductivity,  the  density,  the  specific  heats, 
the  index  of  refraction,  the  power  of  rotatory  polar- 
ization, the  colour,  or  the  absorption  spectrum, 
the  additive  property  will  everywhere  be  found  in 
the  solution. 

The  hypothesis,  so  contested  at  the  outset  by  the 
chemists,  is,  moreover,  assuring  its  triumph  by  im- 
portant conquests  in  the  domain  of  chemistry  itself. 
It  permits  us  to  give  a  vivid  explanation  of  chemical 
reaction,  and  for  the  old  motto  of  the  chemists, 
"  Corpora  non  agunt,  nisi  soluta,"  it  substitutes  a 
modern  one,  "  It  is  especially  the  ions  which  react." 


164    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

Thus,  for  example,  all  salts  of  iron,  which  contain 
iron  in  the  state  of  ions,  give  similar  reactions ;  but 
salts  such  as  ferrocyanide  of  potassium,  in  which 
iron  does  not  play  the  part  of  an  ion,  never  give  the 
characteristic  reactions  of  iron. 

Professor  Ostwald  and  his  pupils  have  drawn  from 
the  hypothesis  of  Arrhenius  manifold  consequences 
which  have  been  the  cause  of  considerable  progress 
in  physical  chemistry.  Professor  Ostwald  has  shown, 
in  particular,  how  this  hypothesis  permits  the  quanti- 
tative calculation  of  the  conditions  of  equilibrium 
of  electrolytes  and  solutions,  and  especially  of  the 
phenomena  of  neutralization.  If  a  dissolved  salt  is 
partly  dissociated  into  ions,  this  solution  must  be 
limited  by  an  equilibrium  between  the  non-dis- 
sociated molecule  and  the  two  ions  resulting  from 
the  dissociation ;  and,  assimilating  the  phenomenon 
to  the  case  of  gases,  we  may  take  for  its  study  the 
laws  of  Gibbs  and  of  Guldberg  and  Waage.  The 
results  are  generally  very  satisfactory,  and  new 
researches  daily  furnish  new  checks. 

Professor  Nernst,  who  before  gave,  as  has  been 
said,  a  remarkable  interpretation  of  the  diffusion  of 
electrolytes,  has,  in  the  direction  pointed  out  by 
M.  Arrhenius,  developed  a  theory  of  the  entire 
phenomena  of  electrolysis,  which,  in  particular, 
furnishes  a  striking  explanation  of  the  mechanism 
of  the  production  of  electromotive  force  in  galvanic 
batteries. 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    165 

Extending  the  analogy,  already  so  happily  in- 
voked, between  the  phenomena  met  with  in  solutions 
and  those  produced  in  gases,  Professor  Nernst 
supposes  that  metals  tend,  as  it  were,  to  vaporize 
when  in  presence  of  a  liquid.  A  piece  of  zinc  intro- 
duced, for  example,  into  pure  water  gives  birth  to 
a  few  metallic  ions.  These  ions  become  positively 
charged,  while  the  metal  naturally  takes  an  equal 
charge,  but  of  contrary  sign.  Thus  the  solution 
and  the  metal  are  both  electrified ;  but  this  sort  of 
vaporization  is  hindered  by  electrostatic  attraction, 
and  as  the  charges  borne  by  the  ions  are  considerable, 
an  equilibrium  will  be  established,  although  the 
number  of  ions  which  enter  the  solution  will  be  very 
small. 

If  the  liquid,  instead  of  being  a  solvent  like  pure 
water,  contains  an  electrolyte,  it  already  contains 
metallic  ions,  the  osmotic  pressure  of  which  will 
be  opposite  to  that  of  the  solution.  Three  cases 
may  then  present  themselves — either  there  will  be 
equilibrium,  or  the  electrostatic  attraction  will 
oppose  itself  to  the  pressure  of  solution  and  the 
metal  will  be  negatively  charged,  or,  finally,  the 
attraction  will  act  in  the  same  direction  as  the 
pressure,  and  the  metal  will  become  positively 
and  the  solution  negatively  charged.  Developing 
this  idea,  Professor  Nernst  calculates,  by  means  of 
the  action  of  the  osmotic  pressures,  the  variations  of 
energy  brought  into  play  and  the  value  of  the 


1 66    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

differences  of  potential  by  the  contact  of  the  elec- 
trodes and  electrolytes.  He  deduces  this  from  the 
electromotive  force  of  a  single  battery  cell  which  be- 
comes thus  connected  with  the  values  of  the  osmotic 
pressures,  or,  if  you  will,  thanks  to  the  relation 
discovered  by  Van  t'  Hoff,  with  the  concentrations. 
Some  particularly  interesting  electrical  phenomena 
thus  become  connected  with  an  already  very  im- 
portant group,  and  a  new  bridge  is  built  which 
unites  two  regions  long  considered  foreign  to  each 
other. 

The  recent  discoveries  on  the  phenomena  produced 
in  gases  when  rendered  conductors  of  electricity 
almost  force  upon  us,  as  we  shall  see,  the  idea  that 
there  exist  in  these  gases  electrified  centres  moving 
through  the  field,  and  this  idea  gives  still  greater 
probability  to  the  analogous  theory  explaining  the 
mechanism  of  the  conductivity  of  liquids.  It  will 
also  be  useful,  in  order  to  avoid  confusion,  to  restate 
with  precision  this  notion  of  electrolytic  ions,  and  to 
ascertain  their  magnitude,  charge,  and  velocity. 

The  two  classic  laws  of  Faraday  will  supply  us 
with  important  information.  The  first  indicates  that 
the  quantity  of  electricity  passing  through  the 
liquid  is  proportional  to  the  quantity  of  matter 
deposited  on  the  electrodes.  This  leads  us  at  once 
to  the  consideration  that,  in  any  given  solution,  all 
the  ions  possess  individual  charges  equal  in  absolute 
value. 


SOLUTIONS  &  ELECTROLYTIC  DISSOCIATION    167 

The  second  law  may  be  stated  in  these  terms  : 
an  atom-gramme  of  metal  carries  with  it  into 
electrolysis  a  quantity  of  electricity  proportionate 
to  its  valency.1 

Numerous  experiments  have  made  known  the 
total  mass  of  hydrogen  capable  of  carrying  one 
coulomb,  and  it  will  therefore  be  possible  to 
estimate  the  charge  of  an  ion  of  hydrogen  if  the 
number  of  atoms  of  hydrogen  in  a  given  mass  be 
known.  This  last  figure  is  already  furnished  by 
considerations  derived  from  the  kinetic  theory,  and 
agrees  with  the  one  which  can  be  deduced  from 
the  study  of  various  phenomena.  The  result  is 
that  an  ion  of  hydrogen  having  a  mass  of  1/3  X 
10~24  grammes  bears  a  charge  of  1-3  X  10~20  electro- 
magnetic units ;  and  the  second  law  will  immedi- 
ately enable  the  charge  of  any  other  ion  to  be 
similarly  estimated. 

The  measurements  of  conductivity,  joined  to 
certain  considerations  relating  to  the  differences  of 
concentration  which  appear  round  the  electrode 
in  electrolysis,  allow  the  speed  of  the  ions  to  be 

1  The  valency  or  atomicity  of  an  element  may  be  denned 
as  the  power  it  possesses  of  entering  into  compounds  in  a 
certain  fixed  proportion.  As  hydrogen  is  generally  taken  as 
the  standard,  in  practice  the  valency  of  an  atom  is  the  number 
of  hydrogen  atoms  it  will  combine  with  or  replace.  Thus 
chlorine  and  the  rest  of  the  halogens,  the  atoms  of  which 
combine  with  one  atom  of  hydrogen,  are  called  univalent, 
oxygen  a  bivalent  element,  and  so  on. — ED. 


1  68    THE  NEW   PHYSICS  AND   ITS  EVOLUTION 


calculated.  Thus,  in  a  liquid  containing  ^th  of 
a  hydrogen-ion  per  litre,  the  absolute  speed  of  an 
ion  would  be  ^^h8  °^  a  millimetre  per  second  in  a 
field  where  the  fall  of  potential  would  be  1  volt  per 
centimetre.  Sir  Oliver  Lodge,  who  has  made  direct 
experiments  to  measure  this  speed,  has  obtained  a 
figure  very  approximate  to  this.  This  value  is  very 
small  compared  to  that  which  we  shall  meet  with 
in  gases. 

Another  consequence  of  the  laws  of  Faraday,  to 
which,  as  early  as  1881,  Helmholtz  drew  attention, 
may  be  considered  as  the  starting-point  of  certain 
new  doctrines  we  shall  come  across  later. 

Helmholtz  says  :  "  If  we  accept  the  hypothesis 
that  simple  bodies  are  composed  of  atoms,  we  are 
obliged  to  admit  that,  in  the  same  way,  electricity, 
whether  positive  or  negative,  is  composed  of  element- 
ary parts  which  behave  like  atoms  of  electricity." 

The  second  law  seems,  in  fact,  analogous  to  the 
law  of  multiple  proportions  in  chemistry,  and  it 
shows  us  that  the  quantities  of  electricity  carried 
vary  from  the  simple  to  the  double  or  treble, 
according  as  it  is  a  question  of  a  uni-,  bi-,  or  tri- 
valent  metal  ;  and  as  the  chemical  law  leads  up  to 
the  conception  of  the  material  atom,  so  does  the 
electrolytic  law  suggest  the  idea  of  an  electric  atom. 


CHAPTEK  VI 
THE  ETHER 

§  1.  THE  LUMINIFEROUS  ETHER 

IT  is  in  the  works  of  Descartes  that  we  find  the 
first  idea  of  attributing  those  physical  phenomena 
which  the  properties  of  matter  fail  to  explain  to 
some  subtle  matter  which  is  the  receptacle  of  the 
energy  of  the  universe. 

In  our  times  this  idea  has  had  extraordinary 
luck.  After  having  been  eclipsed  for  two  hundred 
years  by  the  success  of  the  immortal  synthesis  of 
Newton,  it  gained  an  entirely  new  splendour  with 
Fresnel  and  his  followers.  Thanks  to  their  admir- 
able discoveries,  the  first  stage  seemed  accomplished, 
the  laws  of  optics  were  represented  by  a  single  hypo- 
thesis, marvellously  fitted  to  allow  us  to  anticipate 
unknown  phenomena,  and  all  these  anticipations  were 
subsequently  fully  verified  by  experiment.  But  the 
researches  of  Faraday,  Maxwell,  and  Hertz  authorized 
still  greater  ambitions;  and  it  really  seemed  that 
this  medium,  to  which  it  was  agreed  to  give  the 


170    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

ancient  name  of  ether,  and  which  had  already 
explained  light  and  radiant  heat,  would  also  be 
sufficient  to  explain  electricity.  Thus  the  hope 
began  to  take  form  that  we  might  succeed  in  demon- 
strating the  unity  of  all  physical  forces.  It  was 
thought  that  the  knowledge  of  the  laws  relating  to 
the  inmost  movements  of  this  ether  might  give  us  the 
key  to  all  phenomena,  and  might  make  us  acquainted 
with  the  method  in  which  energy  is  stored  up, 
transmitted,  and  parcelled  out  in  its  external 
manifestations. 

We  cannot  study  here  all  the  problems  which 
are  connected  with  the  physics  of  the  ether.  To 
do  this  a  complete  treatise  on  optics  would  have 
to  be  written  and  a  very  lengthy  one  on  electricity. 
I  shall  simply  endeavour  to  show  rapidly  how  in 
the  last  few  years  the  ideas  relative  to  the  con- 
stitution of  this  ether  have  evolved,  and  we 
shall  see  if  it  be  possible  without  self-delusion  to 
imagine  that  a  single  medium  can  really  allow  us 
to  group  all  the  known  facts  in  one  comprehensive 
arrangement. 

As  constructed  by  Fresnel,  the  hypothesis  of  the 
luminous  ether,  which  had  so  great  a  struggle  at 
the  outset  to  overcome  the  stubborn  resistance  of 
the  partisans  of  the  then  classic  theory  of  emission, 
seemed,  on  the  contrary,  to  possess  in  the  sequel 
an  unshakable  strength.  Lame",  though  a  prudent 
mathematician,  wrote :  "  The  existence  of  the  ethereal 


THE   ETHER  171 

fluid  is  incontestably  demonstrated  by  the  propagation 
of  light  through  the  planetary  spaces,  and  by  the 
explanation,  so  simple  and  so  complete,  of  the 
phenomena  of  diffraction  in  the  wave  theory  of 
light " ;  and  he  adds :  "  The  laws  of  double  refraction 
prove  with  no  less  certainty  that  the  ether  exists  in 
all  diaphanous  media."  Thus  the  ether  was  no  longer 
an  hypothesis,  but  in  some  sort  a  tangible  reality. 
But  the  ethereal  fluid  of  which  the  existence  was 
thus  proclaimed  has  some  singular  properties. 

Were  it  only  a  question  of  explaining  rectilinear 
propagation,  reflexion,  refraction,  diffraction,  and 
interferences  notwithstanding  grave  difficulties  at 
the  outset  and  the  objections  formulated  by  Laplace 
and  Poisson  (some  of  which,  though  treated  some- 
what lightly  at  the  present  day,  have  not  lost  all 
value),  we  should  be  under  no  obligation  to  make 
any  hypothesis  other  than  that  of  the  undulations 
of  an  elastic  medium,  without  deciding  in  advance 
anything  as  to  the  nature  and  direction  of  the 
vibrations. 

This  medium  would,  naturally — since  it  exists  in 
what  we  call  the  void — be  considered  as  imponderable. 
It  may  be  compared  to  a  fluid  of  negligible  mass — 
since  it  offers  no  appreciable  resistance  to  the  motion 
of  the  planets — but  is  endowed  with  an  enormous 
elasticity,  because  the  velocity  of  the  propagation  of 
light  is  considerable.  It  must  be  capable  of  pene- 
trating into  all  transparent  bodies,  and  of  retaining 


172    THE   NEW   PHYSICS  AND   ITS   EVOLUTION 

there,  so  to  speak,  a  constant  elasticity,  but  must 
there  become  condensed,  since  the  speed  of  propaga- 
tion in  these  bodies  is  less  than  in  a  vacuum.  Such 
properties  belong  to  no  material  gas,  even  the  most 
rarefied,  but  they  admit  of  no  essential  contradiction, 
and  that  is  the  important  point.1 

It  was  the  study  of  the  phenomena  of  polariza- 
tion which  led  Fresnel  to  his  bold  conception  of 
transverse  vibrations,  and  subsequently  induced  him 
to  penetrate  further  into  the  constitution  of  the  ether. 
We  know  the  experiment  of  Arago  on  the  non- 
interference of  polarized  rays  in  rectangular  planes. 
While  two  systems  of  waves,  proceeding  from  the 
same  source  of  natural  light  and  propagating  them- 
selves in  nearly  parallel  directions,  increase  or 
become  destroyed  according  to  whether  the  nature 
of  the  superposed  waves  are  of  the  same  or  of 
contrary  signs,  the  waves  of  the  rays  polarized  in 
perpendicular  planes,  on  the  other  hand,  can  never 
interfere  with  each  other.  Whatever  the  difference 

1  Since  this  was  written,  however,  men  of  science  have 
become  less  unanimous  than  they  formerly  were  on  this  point. 
The  veteran  chemist  Professor  Mendeleeff  has  given  reasons 
for  thinking  that  the  ether  is  an  inert  gas  with  an  atomic 
weight  a  million  times  less  than  that  of  hydrogen,  and  a 
velocity  of  2250  kilometres  per  second  (Principles  of  Chemistry, 
Eng.  ed.,  1905,  vol.  ii.  p.  526).  On  the  other  hand,  the  well- 
known  physicist  Dr  A.  H.  Bucherer,  speaking  at  the  Natur- 
forscherversammlung,  held  at  Stuttgart  in  1906,  declared  his 
disbelief  in  the  existence  of  the  ether,  which  he  thought  could 
not  be  reconciled  at  once  with  the  Maxwellian  theory  and  the 
known  facts. — ED. 


THE   ETHER  173 

of  their  course,  the  intensity  of  the  light  is  always 
the  sum  of  the  intensity  of  the  two  rays. 

Fresnel  perceived  that  this  experiment  absolutely 
compels  us  to  reject  the  hypothesis  of  longitudinal 
vibrations  acting  along  the  line  of  propagation  in 
the  direction  of  the  rays.  To  explain  it,  it  must 
of  necessity  be  admitted,  on  the  contrary,  that  the 
vibrations  are  transverse  and  perpendicular  to  the 
ray.  Verdet  could  say,  in  all  truth,  "It  is  not 
possible  to  deny  the  transverse  direction  of  luminous 
vibrations,  without  at  the  same  time  denying  that 
light  consists  of  an  undulatory  movement." 

Such  vibrations  do  not  and  cannot  exist  in  any 
medium  resembling  a  fluid.  The  characteristic  of 
a  fluid  is  that  its  different  parts  can  displace  them- 
selves with  regard  to  one  another  without  any 
reaction  appearing  so  long  as  a  variation  of  volume 
is  not  produced.  There  certainly  may  exist,  as  we 
have  seen,  certain  traces  of  rigidity  in  a  liquid,  but  we 
cannot  conceive  such  a  thing  in  a  body  infinitely  more 
subtle  than  rarefied  gas.  Among  material  bodies,  a 
solid  alone  really  possesses  the  rigidity  sufficient  for 
the  production  within  it  of  transverse  vibrations  and 
for  their  maintenance  during  their  propagation. 

Since  we  have  to  attribute  such  a  property  to  the 
ether,  we  may  add  that  on  this  point  it  resembles  a 
solid,  and  Lord  Kelvin  has  shown  that  this  solid 
would  be  much  more  rigid  than  steel.  This  con- 
clusion produces  great  surprise  in  all  who  hear  it 


174    THE  NEW   PHYSICS   AND  ITS  EVOLUTION 

for  the  first  time,  and  it  is  not  rare  to  hear  it 
appealed  to  as  an  argument  against  the  actual 
existence  of  the  ether.  It  does  not  seem,  however, 
that  such  an  argument  can  be  decisive.  There  is  no 
reason  for  supposing  that  the  ether  ought  to  be  a 
sort  of  extension  of  the  bodies  we  are  accustomed 
to  handle.  Its  properties  may  astonish  our  ordinary 
way  of  thinking,  but  this  rather  unscientific  aston- 
ishment is  not  a  reason  for  doubting  its  existence. 
Eeal  difficulties  would  appear  only  if  we  were  led  to 
attribute  to  the  ether,  not  singular  properties  which 
are  seldom  found  united  in  the  same  substance,  but 
properties  logically  contradictory.  In  short,  how- 
ever odd  such  a  medium  may  appear  to  us,  it  cannot 
be  said  that  there  is  any  absolute  incompatibility 
between  its  attributes. 

It  would  even  be  possible,  if  we  wished,  to  sug- 
gest images  capable  of  representing  these  contrary 
appearances.  Various  authors  have  done  so.  Thus, 
M.  Boussinesq  assumes  that  the  ether  behaves  like 
a  very  rarefied  gas  in  respect  of  the  celestial  bodies, 
because  these  last  move,  while  bathed  in  it,  in  all 
directions  and  relatively  slowly,  while  they  permit  it 
to  retain,  so  to  speak,  its  perfect  homogeneity.  On 
the  other  hand,  its  own  undulations  are  so  rapid 
that  so  far  as  they  are  concerned  the  conditions 
become  very  different,  and  its  fluidity  has,  one  might 
say,  no"  longer  the  time  to  come  in.  Hence  its 
rigidity  alone  appears. 


THE   ETHER  175 

Another  consequence,  very  important  in  principle, 
of  the  fact  that  vibrations  of  light  are  transverse, 
has  been  well  put  in  evidence  by  Fresnel.  He  showed 
how  we  have,  in  order  to  understand  the  action  which 
excites  without  condensation  the  sliding  of  successive 
layers  of  the  ether  during  the  propagation  of  a 
vibration,  to  consider  the  vibrating  medium  as  being 
composed  of  molecules  separated  by  finite  distances. 
Certain  authors,  it  is  true,  have  proposed  theories  in 
which  the  action  at  a  distance  of  these  molecules  are 
replaced  by^a£^^is_ofcontact  between  parallele- 
pipeds sliding  over  one  another;  but,  at.  bottom, 
these  two  points  of  view  both  lead  us  to  conceive  the 
ether  as  a  discontinuous  medium,  like  matter  itself. 
The  ideas  gathered  from  the  most  recent  experiments 
also  bring  us  to  the  same  conclusion. 

§  2.  RADIATIONS 

In  the  ether  thus  constituted  there  are  therefore 
propagated  transverse  vibrations,  regarding  which  all 
experiments  in  optics  furnish  very  precise  informa- 
tion. The  amplitude  of  these  vibrations  is  exceed- 
ingly small,  even  in  relation  to  the  wave-length, 
small  as  these  last  are.  If,  in  fact,  the  amplitude  of 
the  vibrations  acquired  a  noticeable  value  in  com- 
parison with  the  wave-length,  the  speed  of  propagation 
should  increase  with  the  amplitude.  Yet,  in  spite 
of  some  curious  experiments  which  seem  to  establish 
that  the  speed  of  light  does  alter  a  little  with  its 


176     THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

intensity,  we  have  reason  to  believe  that,  as  regards 
light,  the  amplitude  of  the  oscillations  in  relation 
to  the  wave-length  is  incomparably  less  than  in  the 
case  of  sound. 

It  has  become  the  custom  to  characterise  each 
vibration  by  the  path  which  the  vibratory  move- 
ment traverses  during  the  space  of  a  vibration — 
by  the  length  of  wave,  in  a  word — rather  than  by 
the  duration  of  the  vibration  itself.  To  measure 
wave-lengths,  the  methods  must  be  employed  to 
which  I  have  already  alluded  on  the  subject  of 
measurements  of  length.  Professor  Michelson,  on  the 
one  hand,  and  MM.  Perot  and  Fabry,  on  the  other, 
have  devised  exceedingly  ingenious  processes,  which 
have  led  to  results  of  really  unhoped-for  precision. 
The  very  exact  knowledge  also  of  the  speed  of 
the  propagation  of  light  allows  the  duration  of  a 
vibration  to  be  calculated  when  once  the  wave-length 
is  known.  It  is  thus  found  that,  in  the  case  of 
visible  light,  the  number  of  the  vibrations  from 
the  end  of  the  violet  to  the  infra-red  varies 
from  four  hundred  to  two  hundred  billions  per 
second.  This  gamut  is  not,  however,  the  only  one 
the  ether  can  give.  For  a  long  time  we  have  known 
ultra-violet  radiations  still  more  rapid,  and,  on  the 
other  hand,  infra-red  ones  more  slow,  while  in  the 
last  few  years  the  field  of  known  radiations  has  been 
singularly  extended  in  both  directions. 

It  is  to  M,  liubens  an4  his  fellow-workers  that 


THE  ETHER  177 

are  due  the  most  brilliant  conquests  in  the  matter 
of  great  wave-lengths.  He  had  remarked  that,  in 
their  study,  the  difficulty  of  research  proceeds  from 
the  fact  that  the  extreme  waves  of  the  infra-red 
spectrum  only  contain  a  small  part  of  the  total 
energy  emitted  by  an  incandescent  body  ;  so  that  if, 
for  the  purpose  of  study,  they  are  further  dispersed 
by  a  prism  or  a  grating,  the  intensity  at  any  one 
point  becomes  so  slight  as  to  be  no  longer  observ- 
able. His  original  idea  was  to  obtain,  without  prism 
or  grating,  a  homogeneous  pencil  of  great  wave-length 
sufficiently  intense  to  be  examined.  For  this  purpose 
the  radiant  source  used  was  a  strip  of  platinum 
covered  with  fluorine  or  powdered  quartz,  which 
emits  numerous  radiations  close  to  two  bands  of 
linear  absorption  in  the  absorption  spectra  of  fluorine 
and  quartz,  one  of  which  is  situated  in  the  infra- 
red. The  radiations  thus  emitted  are  several  times 
reflected  on  fluorine  or  on  quartz,  as  the  case  may 
be ;  and  as,  in  proximity  to  the  bands,  the  absorption 
is  of  the  order  of  that  of  metallic  bodies  for  lumin- 
ous rays,  we  no  longer  meet  in  the  pencil  several 
times  reflected  or  in  the  rays  remaining  after  this 
kind  of  filtration,  with  any  but  radiations  of  great 
wave-length.  Thus,  for  instance,  in  the  case  of  the 
quartz,  in  the  neighbourhood  of  a  radiation  corre- 
sponding to  a  wave-length  of  8*5  /x,  the  absorption 
is  thirty  times  greater  in  the  region  of  the  band 

than  in  the  neighbouring  region,  and  consequently, 

12 


i;8    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

after  three  reflexions,  while  the  corresponding 
radiations  will  not  have  been  weakened,  the 
neighbouring  waves  will  be  so,  on  the  contrary,  in 
the  proportion  of  1  to  27,000. 

With  mirrors  of  rock  salt  and  of  sylvine  *  there 
have  been  obtained,  by  taking  an  incandescent  gas 
light  (Auer)  as  source,  radiations  extending  as  far  as 
70  /UL  ;  and  these  last  are  the  greatest  wave-lengths 
observed  in  optical  phenomena.  These  radiations 
are  largely  absorbed  by  the  vapour  of  water,  and  it 
is  no  doubt  owing  to  this  absorption  that  they  are 
not  found  in  the  solar  spectrum.  On  the  other 
hand,  they  easily  pass  through  gutta-percha,  india- 
rubber,  and  insulating  substances  in  general. 

At  the  opposite  end  of  the  spectrum  the  know- 
ledge of  the  ultra-violet  regions  has  been  greatly 
extended  by  the  researches  of  Lenard.  These 
extremely  rapid  radiations  have  been  shown  by 
that  eminent  physicist  to  occur  in  the  light  of  the 
electric  sparks  which  flash  between  two  metal  points, 
and  which  are  produced  by  a  large  induction  coil 
with  condenser  and  a  Wehnelt  break.  Professor 
Schumann  has  succeeded  in  photographing  them  by 
depositing  bromide  of  silver  directly  on  glass  plates 
without  fixing  it  with  gelatine ;  and  he  has,  by  the 
same  process,  photographed  in  the  spectrum  of 
hydrogen  a  ray  with  a  wave-length  of  only  0*1  JUL. 

1  A  natural  chlorate  of  potassium,  generally  of  volcanic 
origin. — ED. 


THE  ETHER  179 

The  spectroscope  was  formed  entirely  of  fluor-spar, 
and  a  vacuum  had  been  created  in  it,  for  these 
radiations  are  extremely  absorbable  by  the  air. 

Notwithstanding  the  extreme  smallness  of  the 
luminous  wave-lengths,  it  has  been  possible,  after 
numerous  fruitless  trials,  to  obtain  stationary  waves 
analogous  to  those  which,  in  the  case  of  sound,  are 
produced  in  organ  pipes.  The  marvellous  applica- 
tion M.  Lippmann  has  made  of  these  waves  to 
completely  solve  the  problem  of  photography  in 
colours  is  well  known.  This  discovery,  so  important 
in  itself  and  so  instructive,  since  it  shows  us  how 
the  most  delicate  anticipations  of  theory  may  be 
verified  in  all  their  consequences,  and  lead  the 
physicist  to  the  solution  of  the  problems  occurring 
in  practice,  has  justly  become  popular,  and  there  is, 
therefore,  no  need  to  describe  it  here  in  detail. 

Professor  Wiener  obtained  stationary  waves  some 
little  while  before  M.  Lippmann's  discovery,  in  a  layer 
of  a  sensitive  substance  having  a  grain  sufficiently 
small  in  relation  to  the  length  of  wave.  His  aim 
was  to  solve  a  question  of  great  importance  to  a 
complete  knowledge  of  the  ether.  Fresnel  founded 
his  theory  of  double  refraction  and  reflexion  by 
transparent  surfaces,  on  the  hypothesis  that  the 
vibration  of  a  ray  of  polarized  light  is  perpen- 
dicular to  the  plane  of  polarization.  But  Neumann 
has  proposed,  on  the  contrary,  a  theory  in  which 
he  recognizes  that  the  luminous  vibration  is  in  this 


i8o    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

very  plane.  He  rather  supposes,  in  opposition  to 
Fresnel's  idea,  that  the  density  of  the  ether  remains 
the  same  in  all  media,  while  its  coefficient  of 
elasticity  is  variable. 

Very  remarkable  experiments  on  dispersion  by  M. 
Carvallo  prove  indeed  that  the  idea  of  Fresnel  was, 
if  not  necessary  for  us  to  adopt,  at  least  the  more 
probable  of  the  two ;  but  apart  from  this  indication, 
and  contrary  to  the  hypothesis  of  Neumann,  the  two 
theories,  from  the  point  of  view  of  the  explanation 
of  all  known  facts,  really  appear  to  be  equivalent. 
Are  we  then  in  presence  of  two  mechanical  explana- 
tions, different  indeed,  but  nevertheless  both  adaptable 
to  all  the  facts,  and  between  which  it  will  always  be 
impossible  to  make  a  choice  ?  Or,  on  the  contrary, 
shall  we  succeed  in  realising  an  experimentum  crucis, 
an  experiment  at  the  point  where  the  two  theories 
cross,  which  will  definitely  settle  the  question  ? 

Professor  Wiener  thought  he  could  draw  from  his 
experiment  a  firm  conclusion  on  the  point  in  dispute. 
He  produced  stationary  waves  with  light  polarized 
at  an  angle  of  450,1  and  established  that,  when  light 
is  polarized  in  the  plane  of  incidence,  the  fringes 
persist ;  but  that,  on  the  other  hand,  they  disappear 
when  the  light  is  polarized  perpendicularly  to  this 

1  That  is  to  say,  he  reflected  the  beam  of  polarized  light 
by  a  mirror  placed  at  that  angle.  See  Turpain,  Lepons 
elementaires  de  Physique,  t.  ii.  p.  311,  for  details  of  the 
experiment. — ED. 


THE  ETHER  181 

plane.  If  it  be  admitted  that  a  photographic  impres- 
sion results  from  the  active  force  of  the  vibratory 
movement  of  the  ether,  the  question  is,  in  fact,  com- 
pletely elucidated,  and  the  discrepancy  is  abolished 
in  Fresnel's  favour. 

M.  H.  Poincare  has  pointed  out,  however,  that  we 
know  nothing  as  to  the  mechanism  of  the  photographic 
impression.  We  cannot  consider  it  evident  that  it 
is  the  kinetic  energy  of  the  ether  which  produces 
the  decomposition  of  the  sensitive  salt ;  and  if,  on 
the  contrary,  we  suppose  it  to  be  due  to  the  poten- 
tial energy,  all  the  conclusions  are  reversed,  and 
Neumann's  idea  triumphs. 

Eecently  a  very  clever  physicist,  M.  Cotton, 
especially  known  for  his  skilful  researches  in  the 
domain  of  optics,  has  taken  up  anew  the  study  of 
stationary  waves.  He  has  made  very  precise 
quantitative  experiments,  and  has  demonstrated,  in 
his  turn,  that  it  is  impossible,  even  with  spherical 
waves,  to  succeed  in  determining  on  which  of  the 
two  vectors  which  have  to  be  regarded  in  all  theories 
of  light  on  the  subject  of  polarization  phenomena 
the  luminous  intensity  and  the  chemical  action 
really  depend.  This  question,  therefore,  no  longer 
exists  for  those  physicists  who  admit  that  luminous 
vibrations  are  electrical  oscillations.  Whatever, 
then,  the  hypothesis  formed,  whether  it  be  electric 
force  or,  on  the  contrary,  magnetic  force  which  we 
place  in  the  plane  of  polarization,  the  mode  of  pro- 


1 82    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

pagation  foreseen  will  always  be  in  accord  with  the 
facts  observed. 

§  3.  THE  ELECTROMAGNETIC  ETHER 
The  idea  of  attributing  the  phenomena  of  elec- 
tricity to  perturbations  produced  in  the  medium 
which  transmits  the  light  is  already  of  old  standing ; 
and  the  physicists  who  witnessed  the  triumph  of 
Fresnel's  theories  could  not  fail  to  conceive  that 
this  fluid,  which  fills  the  whole  of  space  and  pene- 
trates into  all  bodies,  might  also  play  a  preponder- 
ant part  in  electrical  actions.  Some  even  formed  too 
hasty  hypotheses  on  this  point ;  for  the  hour  had 
not  arrived  when  it  was  possible  to  place  them 
on  a  sufficiently  sound  basis,  and  the  known  facts 
were  not  numerous  enough  to  give  the  necessary 
precision. 

The  founders  of  modern  electricity  also  thought 
it  wiser  to  adopt,  with  regard  to  this  science,  the 
attitude  taken  by  Newton  in  connection  with 
gravitation :  "In  the  first  place  to  observe  facts, 
to  vary  the  circumstances  of  these  as  much  as 
possible,  to  accompany  this  first  work  by  precise 
measurements  in  order  to  deduce  from  them  general 
laws  founded  solely  on  experiment,  and  to  deduce 
from  these  laws,  independently  of  all  hypotheses  on 
the  nature  of  the  forces  producing  the  phenomena, 
the  mathematical  value  of  these  forces — that  is  to 
say,  the  formula  representing  them.  Such  was  the 


THE  ETHER  183 

system  pursued  by  Newton.  It  has,  in  general, 
been  adopted  in  France  by  the  scholars  to  whom 
physics  owe  the  great  progress  made  of  late  years, 
and  it  has  served  as  my  guide  in  all  my  researches 
on  electrodynamic  phenomena.  ...  It  is  for  this 
reason  that  I  have  avoided  speaking  of  the  ideas  I 
may  have  on  the  nature  of  the  cause  of  the  force 
emanating  from  voltaic  conductors." 

Thus  did  Ampere  express  himself.  The  illustrious 
physicist  rightly  considered  the  results  obtained  by 
him  through  following  this  wise  method  as  worthy  of 
comparison  with  the  laws  of  attraction ;  but  he  knew 
that  when  this  first  halting-place  was  reached  there 
was  still  further  to  go,  and  that  the  evolution  of 
ideas  must  necessarily  continue. 

"  With  whatever  physical  cause,"  he  adds,  "we  may 
wish  to  connect  the  phenomena  produced  by  electro- 
dynamic  action,  the  formula  I  have  obtained  will 
always  remain  the  expression  of  the  facts,"  and  he 
explicitly  indicated  that  if  one  could  succeed  in 
deducing  his  formula  from  the  consideration  of  the 
vibrations  of  a  fluid  distributed  through  space,  an 
enormous  step  would  have  been  taken  in  this 
department  of  physics.  He  added,  however,  that 
this  research  appeared  to  him  premature,  and  would 
change  nothing  in  the  results  of  his  work,  since,  to 
accord  with  facts,  the  hypothesis  adopted  would 
always  have  to  agree  with  the  formula  which  exactly 
represents  them. 


1 84    THE   NEW   PHYSICS  AND   ITS  EVOLUTION 

It  is  not  devoid  of  interest  to  observe  that  Ampere 
himself,  notwithstanding  his  caution,  really  formed 
some  hypotheses,  and  recognized  that  electrical 
phenomena  were  governed  by  the  laws  of  mechanics. 
Yet  the  principles  of  Newton  then  appeared  to  be 
unshakable. 

Faraday  was  the  first  to  demonstrate,  by  clear 
experiment,  the  influence  of  the  media  in  electricity 
and  magnetic  phenomena,  and  he  attributed  this 
influence  to  certain  modifications  in  the  ether  which 
these  media  enclose.  His  fundamental  conception 
was  to  reject  action  at  a  distance,  and  to  localize  in 
the  ether  the  energy  whose  evolution  is  the  cause 
of  the  actions  manifested,  as,  for  example,  in  the 
discharge  of  a  condenser. 

Consider  the  barrel  of  a  pump  placed  in  a  vacuum 
and  closed  by  a  piston  at  each  end,  and  let  us  intro- 
duce between  these  a  certain  mass  of  air.  The  two 
pistons,  through  the  elastic  force  of  the  gas,  repel 
each  other  with  a  force  which,  according  to  the  law 
of  Mariotte,  varies  in  inverse  ratio  to  the  distance. 
The  method  favoured  by  Ampere  would  first  of  all 
allow  this  law  of  repulsion  between  the  two  pistons 
to  be  discovered,  even  if  the  existence  of  a  gas 
enclosed  in  the  barrel  of  the  pump  were  unsus- 
pected ;  and  it  would  then  be  natural  to  localize  the 
potential  energy  of  the  system  on  the  surface  of  the 
two  pistons.  But  if  the  phenomenon  is  more  care- 
fully examined,  we  shall  discover  the  presence  of  the 


THE  ETHER  185 

air,  and  we  shall  understand  that  every  part  of  the 
volume  of  this  air  could,  if  it  were  drawn  off  into 
a  recipient  of  equal  volume,  carry  away  with  it  a 
fraction  of  the  energy  of  the  system,  and  that  con- 
sequently this  energy  belongs  really  to  the  air  and 
not  to  the  pistons,  which  are .  there  solely  for  the 
purpose  of  enabling  this  energy  to  manifest  its 
existence. 

Faraday  made,  in  some  sort,  an  equivalent  dis- 
covery when  he  perceived  that  the  electrical  energy 
belongs,  not  to  the  coatings  of  the  condenser,  but 
to  the  dielectric  which  separates  them.  His  auda- 
cious views  revealed  to  him  a  new  world,  but  to 
explore  this  world  a  surer  and  more  patient  method 
was  needed. 

Maxwell  succeeded  in  stating  with  precision 
certain  points  of  Faraday's  ideas,  and  he  gave  them 
the  mathematical  form  which,  often  wrongly,  im- 
presses physicists,  but  which  when  it  exactly  encloses 
a  theory,  is  a  certain  proof  that  this  theory  is  at 
least  coherent  and  logical.1 

The  work  of  Maxwell  is  over-elaborated,  complex, 

1  It  will  no  doubt  be  a  shock  to  those  whom  Professor  Henry 
Armstrong  has  lately  called  the  "mathematically-minded" 
to  find  a  member  of  the  Poincare  family  speaking  disrespect- 
fully of  the  science  they  have  done  so  much  to  illustrate.  One 
may  perhaps  compare  the  expression  in  the  text  with  M. 
Henri  Poincare' s  remark  in  his  last  allocution  to  the  Academic 
des  Sciences,  that  "  Mathematics  are  sometimes  a  nuisance,  and 
even  a  danger,  when  they  induce  us  to  affirm  more  than  we 
know"  (Comptes-rendus,  17th  December  1906). 


1 86    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

difficult  to  read,  and  often  ill-understood,  even  at  the 
present  day.  Maxwell  is  more  concerned  in  discover- 
ing whether  it  is  possible  to  give  an  explanation  of 
electrical  and  magnetic  phenomena  which  shall  be 
founded  on  the  mechanical  properties  of  a  single 
medium,  than  in  stating  this  explanation  in  precise 
terms.  He  is  aware  that  if  we  could  succeed  in 
constructing  such  an  interpretation,  it  would  be 
easy  to  propose  an  infinity  of  others,  entirely  equiva- 
lent from  the  point  of  view  of  the  experimentally 
verifiable  consequences  ;  and  his  especial  ambition 
is  therefore  to  extract  from  the  premises  a  general 
view,  and  to  place  in  evidence  something  which  would 
remain  the  common  property  of  all  the  theories. 

He  succeeded  in  showing  that  if  the  electrostatic 
energy  of  an  electromagnetic  field  be  considered  to 
represent  potential  energy,  and  its  electrodynamic 
the  kinetic  energy,  it  becomes  possible  to  satisfy 
both  the  principle  of  least  action  and  that  of  the 
conservation  of  energy;  from  that  moment — if  we 
eliminate  a  few  difficulties  which  exist  regarding  the 
stability  of  the  solutions — the  possibility  of  finding 
mechanical  explanations  of  electromagnetic  pheno- 
mena must  be  considered  as  demonstrated.  He 
thus  succeeded,  moreover,  in  stating  precisely  the 
notion  of  two  electric  and  magnetic  fields  which  are 
produced  in  all  points  of  space,  and  which  are  strictly 
inter-connected,  since  the  variation  of  the  one  imme- 
diately and  compulsorily  gives  birth  to  the  other. 


THE  ETHER  187 

From  this  hypothesis  he  deduced  that,  in  the 
medium  where  this  energy  is  localized,  an  electro- 
magnetic wave  is  propagated  with  a  velocity  equal 
to  the  relation  of  the  units  of  electric  mass  in  the 
electromagnetic  and  electrostatic  systems.  Now, 
experiments  made  known  since  his  time  have  proved 
that  this  relation  is  numerically  equal  to  the  speed 
of  light,  and  the  more  precise  experiments  made  in 
consequence — among  which  should  be  cited  the 
particularly  careful  ones  of  M.  Max  Abraham — have 
only  rendered  the  coincidence  still  more  complete. 

It  is  natural  henceforth  to  suppose  that  this 
medium  is  identical  with  the  luminous  ether,  and 
that  a  luminous  wave  is  an  electromagnetic  wave 
— that  is  to  say,  a  succession  of  alternating  currents, 
which  exist  in  the  dielectric  and  even  in  the  void, 
and  possess  an  enormous  frequency,  inasmuch  as 
they  change  their  direction  thousands  of  billions  of 
times  per  second,  and  by  reason  of  this  frequency 
produce  considerable  induction  effects.  Maxwell 
did  not  admit  the  existence  of  open  currents. 
To  his  mind,  therefore,  an  electrical  vibration  could 
not  produce  condensations  of  electricity.  It  was, 
in  consequence,  necessarily  transverse,  and  thus 
coincided  with  the  vibration  of  Fresnel ;  while  the 
corresponding  magnetic  vibration  was  perpendicular 
to  it,  and  would  coincide  with  the  luminous  vibration 
of  Neumann. 

Maxwell's  theory  thus   establishes   a  close  corre- 


1 88    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

lation  between  the  phenomena  of  the  luminous  and 
those  of  the  electromagnetic  waves,  or,  we  might 
even  say,  the  complete  identity  of  the  two.  But  it 
does  not  follow  from  this  that  we  ought  to  regard 
the  variation  of  an  electric  field  produced  at  some 
one  point  as  necessarily  consisting  of  a  real  displace- 
ment of  the  ether  round  that  point,  The  idea  of 
thus  bringing  electrical  phenomena  back  to  the 
mechanics  of  the  ether  is  not,  then,  forced  upon  us, 
and  the  contrary  idea  even  seems  more  probable.  It 
is  not  the  optics  of  Fresnel  which  absorbs  the  science 
of  electricity,  it  is  rather  the  optics  which  is  swallowed 
up  by  a  more  general  theory.  The  attempts  of  popu- 
larizers  who  endeavour  to  represent,  in  all  their  details, 
the  mechanism  of  the  electric  phenomena,  thus  appear 
vain  enough,  and  even  puerile.  It  is  useless  to  find 
out  to  what  material  body  the  ether  may  be  com- 
pared, if  we  content  ourselves  with  seeing  in  it  a 
medium  of  which,  at  every  point,  two  vectors  define 
the  properties. 

For  a  long  time,  therefore,  we  could  remark  that 
the  theory  of  Fresnel  simply  supposed  a  medium  in 
which  something  periodical  was  propagated,  without 
its  being  necessary  to  admit  this  something  to  be  a 
movement ;  but  we  had  to  wait  not  only  for  Maxwell, 
but  also  for  Hertz,  before  this  idea  assumed  a  really 
scientific  shape.  Hertz  insisted  on  the  fact  that  the 
six  equations  of  the  electric  field  permit  all  the  pheno- 
mena to  be  anticipated  without  its  being  necessary  to 


THE  ETHER  189 

construct  one  hypothesis  or  another,  and  he  put 
these  equations  into  a  very  symmetrical  form,  which 
brings  completely  in  evidence  the  perfect  reciprocity 
between  electrical  and  magnetic  actions.  He  did 
yet  more,  for  he  brought  to  the  ideas  of  Maxwell 
the  most  striking  confirmation  by  his  memorable 
researches  on  electric  oscillations. 

§  4.  ELECTRICAL  OSCILLATIONS 

The  experiments  of  Hertz  are  well  known.  We 
know  how  the  Bonn  physicist  developed,  by  means  of 
oscillating  electric  discharges,  displacement  currents 
and  induction  effects  in  the  whole  of  the  space  round 
the  spark-gap ;  and  how  he  excited  by  induction  at 
some  point  in  a  wire  a  perturbation  which  afterwards 
is  propagated  along  the  wire,  and  how  a  resonator 
enabled  him  to  detect  the  effect  produced. 

The  most  important  point  made  evident  by  the 
observation  of  interference  phenomena  and  subse- 
quently verified  directly  by  M.  Blondlot,  is  that 
the  electromagnetic  perturbation  is  propagated  with 
the  speed  of  light,  and  this  result  condemns  for  ever 
all  the  hypotheses  which  fail  to  attribute  any  part 
to  the  intervening  media  in  the  propagation  of  an 
induction  phenomenon. 

If  the  inducing  action  were,  in  fact,  to  operate 
directly  between  the  inducing  and  the  induced  cir- 
cuits, the  propagation  should  be  instantaneous ;  for  if 
an  interval  were  to  occur  between  the  moment  when 


190    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

the  cause  acted  and  the  one  when  the  effect  was  pro- 
duced, during  this  interval  there  would  no  longer 
be  anything  anywhere,  since  the  intervening  medium 
does  not  come  into  play,  and  the  phenomenon  would 
then  disappear. 

Leaving  on  one  side  the  manifold  but  purely 
electrical  consequences  of  this  and  the  numerous 
researches  relating  to  the  production  or  to  the  pro- 
perties of  the  waves — some  of  which,  those  of  MM. 
Sarrazin  and  de  la  Eive,  Eighi,  Turpain,  Lebedeff, 
Decombe,  Barbillon,  Drude,  Gutton,  Lamotte,  Lecher, 
etc.,  are,  however,  of  the  highest  order — I  shall  only 
mention  here  the  studies  more  particularly  directed 
to  the  establishment  of  the  identity  of  the  electro- 
magnetic and  the  luminous  waves. 

The  only  differences  which  subsist  are  necessarily 
those  due  to  the  considerable  discrepancy  which 
exists  between  the  durations  of  the  periods  of  these 
two  categories  of  waves.  The  length  of  wave 
corresponding  to  the  first  spark-gap  of  Hertz  was 
about  6  metres,  and  the  longest  waves  perceptible 
by  the  retina  are  -fa  of  a  micron.1 

These  radiations  are  so  far  apart  that  it  is  not 
astonishing  that  their  properties  have  not  a  perfect 
similitude.  Thus  phenomena  like  those  of  diffrac- 
tion, which  are  negligible  in  the  ordinary  conditions 
under  which  light  is  observed,  may  here  assume 
a  preponderating  importance.  To  play  the  part,  for 
1  See  note  on  page  27. 


THE  ETHER  191 

example,  with  the  Hertzian  waves,  which  a  mirror 
1  millimetre  square  plays  with  regard  to  light, 
would  require  a  colossal  mirror  which  would  attain 
the  size  of  a  myriametre  square. 

The  efforts  of  physicists  have  to-day,  however, 
filled  up,  in  great  part,  this  interval,  and  from  both 
banks  at  once  they  have  laboured  to  build  a  bridge 
between  the  two  domains.  We  have  seen  how 
Eubens  showed  us  calorific  rays  60  metres  long ;  on 
the  other  hand,  MM.  Lecher,  Bose,  and  Lampa  have 
succeeded,  one  after  the  other,  in  gradually  obtaining 
oscillations  with  shorter  and  shorter  periods.  There 
have  been  produced,  and  are  now  being  studied, 
electromagnetic  waves  of  four  millimetres  ;  and  the 
gap  subsisting  in  the  spectrum  between  the  rays 
left  Undetected  by  sylvine  and  the  radiations  of 
M.  Lampa  now  hardly  comprise  more  than  five 
octaves — that  is  to  say,  an  interval  perceptibly 
equal  to  that  which  separates  the  rays  observed 
by  M.  Eubens  from  the  last  which  are  evident  to 
the  eye. 

The  analogy  then  becomes  quite  close,  and  in  the 
remaining  rays  the  properties,  so  to  speak,  character- 
istic of  the  Hertzian  waves,  begin  to  appear.  For 
these  waves,  as  we  have  seen,  the  most  transparent 
bodies  are  the  most  perfect  electrical  insulators ; 
while  bodies  still  slightly  conducting  are  entirely 
opaque.  The  index  of  refraction  of  these  substances 
tends  in  the  case  of  great  wave-lengths  to  become, 


192    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

as  the  theory  anticipates,  nearly  the  square  root  of 
the  dielectric  constant. 

MM.  Eubens  and  Nichols  have  even  produced 
with  the  waves  which  remain  phenomena  of  electric 
resonance  quite  similar  to  those  which  an  Italian 
scholar,  M.  Garbasso,  obtained  with  electric  waves. 
This  physicist  showed  that,  if  the  electric  waves 
are  made  to  impinge  on  a  flat  wooden  stand,  on 
which  are  a  series  of  resonators  parallel  to  each 
other  and  uniformly  arranged,  these  waves  are 
hardly  reflected  save  in  the  case  where  the  resonators 
have  the  same  period  as  the  spark-gap.  If  the 
remaining  rays  are  allowed  to  fall  on  a  glass 
plate  silvered  and  divided  by  a  diamond  fixed  on 
a  dividing  machine  into  small  rectangles  of  equal 
dimensions,  there  will  be  observed  variations  in 
the  reflecting  power  according  to  the  orientation  of 
the  rectangles,  under  conditions  entirely  comparable 
with  the  experiment  of  Garbasso. 

In  order  that  the  phenomenon  be  produced  it  is 
necessary  that  the  remaining  waves  should  be  pre- 
viously polarized.  This  is  because,  in  fact,  the 
mechanism  employed  to  produce  the  electric  oscilla- 
tions evidently  gives  out  vibrations  which  occur  on 
a  single  plane  and  are  subsequently  polarized. 

We  cannot  therefore  entirely  assimilate  a  radia- 
tion proceeding  from  a  spark-gap  to  a  ray  of 
natural  light.  For  the  synthesis  of  light  to  be 
realized,  still  other  conditions  must  be  complied 


THE  ETHER  193 

with.  During  a  luminous  impression,  the  direction 
and  the  phase  change  millions  of  times  in  the  vibra- 
tion sensible  to  the  retina,  yet  the  damping  of  this 
vibration  is  very  slow.  With  the  Hertzian  oscilla- 
tions all  these  conditions  are  changed — the  damping 
is  very  rapid  but  the  direction  remains  invariable. 

Every  time,  however,  that  we  deal  with  general 
phenomena  which  are  independent  of  these  special 
conditions,  the  parallelism  is  perfect ;  and  with  the 
waves,  we  have  put  in  evidence  the  reflexion, 
refraction,  total  reflexion,  double  reflexion,  rotatory 
polarization,  dispersion,  and  the  ordinary  interfer- 
ences produced  by  rays  travelling  in  the  same  direc- 
tion and  crossing  each  other  at  a  very  acute  angle,  or 
the  interferences  analogous  to  those  which  Wiener 
observed  with  rays  of  the  contrary  direction. 

A  very  important  consequence  of  the  electro- 
magnetic theory  foreseen  by  Maxwell  is  that  the 
luminous  waves  which  fall  on  a  surface  must  exercise 
on  this  surface  a  pressure  equal  to  the  radiant 
energy  which  exists  in  the  unit  of  volume  of  the 
surrounding  space.  M.  Lebedeff  a  few  years  ago 
allowed  a  sheaf  of  rays  from  an  arc  lamp  to  fall  on 
a  deflection  radiometer,1  and  thus  succeeded  in  re- 
vealing the  existence  of  this  pressure.  Its  value  is 

1  By  this  M.  Poincare  appears  to  mean  a  radiometer  in 
which  the  vanes  are  not  entirely  free  to  move  as  in  the  radio- 
meter of  Crookes  but  are  suspended  by  one  or  two  threads 
as  in  the  instrument  devised  by  Professor  Poynting. — ED. 

'3 


i94    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

sufficient,  in  the  case  of  matter  of  little  density  and 
finely  divided,  to  reduce  and  even  change  into 
repulsion  the  attractive  action  exercised  on  bodies 
by  the  sun.  This  is  a  fact  formerly  conjectured  by 
Faye,  and  must  certainly  play  a  great  part  in  the 
deformation  of  the  heads  of  comets. 

More  recently,  MM.  Nichols  and  Hull  have 
undertaken  experiments  on  this  point.  They  have 
measured  not  only  the  pressure,  but  also  the  energy 
of  the  radiation  by  means  of  a  special  bolometer. 
They  have  thus  arrived  at  numerical  verifications 
which  are  entirely  in  conformity  with  the  calcula- 
tions of  Maxwell. 

The  existence  of  these  pressures  may  be  otherwise 
foreseen  even  apart  from  the  electromagnetic  theory, 
by  adding  to  the  theory  of  undulations  the  principles 
of  thermodynamics.  Bartoli,  and  more  recently  Dr 
Larmor,  have  shown,  in  fact,  that  if  these  pressures 
did  not  exist,  it  would  be  possible,  without  any  other 
phenomenon,  to  pass  heat  from  a  cold  into  a  warm 
body,  and  thus  transgress  the  principle  of  Carnot. 

§  5.  THE  X  KAYS 

It  appears  to-day  quite  probable  that  the  X  rays 
should  be  classed  among  the  phenomena  which  have 
their  seat  in  the  luminous  ether.  Doubtless  it  is 
not  necessary  to  recall  here  how,  in  December  1895, 
Kontgen,  having  wrapped  in  black  paper  a  Crookes 
tube  in  action,  observed  that  a  fluorescent  platino- 


THE  ETHER  195 

cyanide  of  barium  screen  placed  in  the  neighbourhood, 
had  become  visible  in  the  dark,  and  that  a  photo- 
graphic plate  had  received  an  impress.  The  rays 
which  come  from  the  tube,  in  conditions  now  well 
known,  are  not  deviated  by  a  magnet,  and,  as  M. 
Curie  and  M.  Sagnac  have  conclusively  shown, 
they  carry  no  electric  charge.  They  are  subject 
to  neither  reflection  nor  refraction,  and  very  precise 
and  very  ingenious  measurements  by  M.  G-ouy  have 
shown  that,  in  their  case,  the  refraction  index  of  the 
various  bodies  cannot  be  more  than  a  millionth 
removed  from  unity. 

We  knew  from  the  outset  that  there  existed  various 
X  rays  differing  from  each  other  as,  for  instance, 
the  colours  of  the  spectrum,  and  these  are  distin- 
guished from  each  other  by  their  unequal  power  of 
passing  through  substances.  M.  Sagnac,  particularly, 
has  shown  that  there  can  be  obtained  a  gradually 
decreasing  scale  of  more  or  less  absorbable  rays,  so 
that  the  greater  part  of  their  photographic  action 
is  stopped  by  a  simple  sheet  of  black  paper.  These 
rays  figure  among  the  secondary  rays  discovered,  as 
is  known,  by  this  ingenious  physicist.  The  X  rays 
falling  on  matter  are  thus  subjected  to  transforma- 
tions which  may  be  compared  to  those  which  the 
phenomena  of  luminescence  produce  on  the  ultra- 
violet rays. 

M.  Benoist  has  founded  on  the  transparency  of 
matter  to  the  rays  a  sure  and  practical  method  of 


I96    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

allowing  them  to  be  distinguished,  and  has  thus  been 
enabled  to  define  a  specific  character  analogous  to 
the  colour  of  the  rays  of  light.  It  is  probable  also 
that  the  different  rays  do  not  transport  individually 
the  same  quantity  of  energy.  We  have  not  yet  ob- 
tained on  this  point  precise  results,  but  it  is  roughly 
known,  since  the  experiments  of  MM.  Eutherford 
and  M'Clung,  what  quantity  of  energy  corresponds 
to  a  pencil  of  X  rays.  These  physicists  have  found 
that  this  quantity  would  be,  on  an  average,  five 
hundred  times  larger  than  that  brought  by  an  analo- 
gous pencil  of  solar  light  to  the  surface  of  the  earth. 
What  is  the  nature  of  this  energy  ?  The  question 
does  not  appear  to  have  been  yet  solved. 

It  certainly  appears,  according  to  Professors  Haga 
and  Wind  and  to  Professor  Sommerfeld,  that  with 
the  X  rays  curious  experiments  of  diffraction  may  be 
produced.  Dr  Barkla  has  shown  also  that  they  can 
manifest  true  polarization.  The  secondary  rays 
emitted  by  a  metallic  surface  when  struck  by  X  rays 
vary,  in  fact,  in  intensity  when  the  position  of 
the  plane  of  incidence  round  the  primary  pencil 
is  changed.  Various  physicists  have  endeavoured 
to  measure  the  speed  of  propagation,  but  it  seems 
more  and  more  probable  that  it  is  very  nearly  that 
of  light.1 

1  See  especially  the  experiments  of  Professor  E.  Marx 
(Vienna),  Annalen  der  PhysiJc,  vol.  xx.  (No.  9  of  1906),  pp. 
677  et  seq.,  which  seem  conclusive  on  this  point. — ED. 


THE  ETHER  197 

I  must  here  leave  out  the  description  of  a  crowd 
of  other  experiments.  Some  very  interesting  re- 
searches by  M.  Brunhes,  M.  Broca,  M.  Colardeau, 
M.  Villard,  in  France,  and  by  many  others  abroad, 
have  permitted  the  elucidation  of  several  interesting 
problems  relative  to  the  duration  of  the  emission  or 
to  the  best  disposition  to  be  adopted  for  the  produc- 
tion of  the  rays.  The  only  point  which  will  detain 
us  is  the  important  question  as  to  the  nature  of 
the  X  rays  themselves ;  the  properties  which  have 
just  been  brought  to  mind  are  those  which  appear 
essential  and  which  every  theory  must  reckon  with. 

The  most  natural  hypothesis  would  be  to  con- 
sider the  rays  as  ultra-violet  radiations  of  very 
short  wave-length,  or  radiations  which  are  in  a 
manner  ultra-ultra-violet.  This  interpretation  can 
still,  at  this  present  moment,  be  maintained,  and  the 
researches  of  MM.  Buisson,  Righi,  Lenard,  and  Merrit 
Stewart  have  even  established  that  rays  of  very  short 
wave-lengths  produce  on  metallic  conductors,  from 
the  point  of  view  of  electrical  phenomena,  effects 
quite  analogous  to  those  of  the  X  rays.  Another 
resemblance  results  also  from  the  experiments  by 
which  M.  Perreau  established  that  these  rays  act 
on  the  electric  resistance  of  selenium.  New  and 
valuable  arguments  have  thus  added  force  to  those 
who  incline  towards  a  theory  which  has  the  merit 
of  bringing  a  new  phenomenon  within  the  pale  of 
phenomena  previously  known. 


i98     THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

Nevertheless  the  shortest  ultra-violet  radiations, 
such  as  those  of  M.  Schumann,  are  still  capable  of 
refraction  by  quartz,  and  this  difference  constitutes, 
in  the  minds  of  many  physicists,  a  serious  enough 
reason  to  decide  them  to  reject  the  more  simple 
hypothesis.  Moreover,  the  rays  of  Schumann  are, 
as  we  have  seen,  extraordinarily  absorbable, — 
so  much  so  that  they  have  to  be  observed  in  a 
vacuum.  The  most  striking  property  of  the  X  rays 
is,  on  the  contrary,  the  facility  with  which  they 
pass  through  obstacles,  and  it  is  impossible  not  to 
attach  considerable  importance  to  such  a  difference. 

Some  attribute  this  marvellous  radiation  to 
longitudinal  vibrations,  which,  as  M.  Duhem  has 
shown,  would  be  propagated  in  dielectric  media  with 
a  speed  equal  to  that  of  light.  But  the  most 
generally  accepted  idea  is  the  one  formulated  from 
the  first  by  Sir  George  Stokes  and  followed  up  by 
Professor  Wiechert.  According  to  this  theory  the  X 
rays  should  be  due  to  a  succession  of  independent 
pulsations  of  the  ether,  starting  from  the  points 
where  the  molecules  projected  by  the  cathode  of 
the  Crookes  tube  meet  the  anticathode.  These 
pulsations  are  not  continuous  vibrations  like  the 
radiations  of  the  spectrum ;  they  are  isolated  and 
extremely  short ;  they  are,  besides,  transverse,  like 
the  undulations  of  light,  and  the  theory  shows 
that  they  must  be  propagated  with  the  speed  of 
light.  They  should  present  neither  refraction  nor 


THE  ETHER  199 

reflection,  but,  under  certain  conditions,  they  may 
be  subject  to  the  phenomena  of  diffraction.  All 
these  characteristics  are  found  in  the  Eontgen  rays. 

Professor  J.  J.  Thomson  adopts  an  analogous  idea, 
and  states  the  precise  way  in  which  the  pulsations 
may  be  produced  at  the  moment  when  the  electrified 
particles  forming  the  cathode  rays  suddenly  strike 
the  anticathode  wall.  The  electromagnetic  induction 
behaves  in  such  a  way  that  the  magnetic  field  is  not 
annihilated  when  the  particle  stops,  and  the  new 
field  produced,  which  is  no  longer  in  equilibrium, 
is  propagated  in  the  dielectric  like  an  electric  pulsa- 
tion. The  electric  and  magnetic  pulsations  excited 
by  this  mechanism  may  give  birth  to  effects  similar 
to  those  of  light.  Their  slight  amplitude,  however, 
is  the  cause  of  there  here  being  neither  refraction  nor 
diffraction  phenomena,  save  in  very  special  conditions. 
If  the  cathode  particle  is  not  stopped  in  zero  time, 
the  pulsation  will  take  a  greater  amplitude,  and  be, 
in  consequence,  more  easily  absorbable  ;  to  this  is 
probably  to  be  attributed  the  differences  which  may 
exist  between  different  tubes  and  different  rays. 

It  is  right  to  add  that  some  authors,  notwith- 
standing the  proved  impossibility  of  deviating  them 
in  a  magnetic  field,  have  not  renounced  the  idea 
of  comparing  them  with  the  cathode  rays.  They 
suppose,  for  instance,  that  the  rays  are  formed  by 
electrons  animated  with  so  great  a  velocity  that 
their  inertia,  conformably  with  theories  which  I  shall 


200     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

examine  later,  no  longer  permit  them  to  be  stopped 
in  their  course ;  this  is,  for  instance,  the  theory 
upheld  by  Mr  Sutherland.  We  know,  too,  that 
to  M.  Gustave  Le  Bon  they  represent  the  extreme 
limit  of  material  things,  one  of  the  last  stages  before 
the  vanishing  of  matter  on  its  return  to  the  ether. 

Everyone  has  heard  of  the  N  rays,  whose  name 
recalls  the  town  of  Nancy,  where  they  were  dis- 
covered. In  some  of  their  singular  properties  they 
are  akin  to  the  X  rays,  while  in  others  they  are 
widely  divergent  from  them. 

M.  Blondlot,  one  of  the  masters  of  contemporary 
physics,  deeply  respected  by  all  who  know  him, 
admired  by  everyone  for  the  penetration  of  his 
mind,  and  the  author  of  works  remarkable  for  the 
originality  and  sureness  of  his  method,  discovered 
them  in  radiations  emitted  from  various  sources, 
such  as  the  sun,  an  incandescent  light,  a  Nernst 
lamp,  and  even  bodies  previously  'exposed  to  the 
sun's  rays.  The  essential  property  which  allows 
them  to  be  revealed  is  their  action  on  a  small  induc- 
tion spark,  of  which  they  increase  the  brilliancy ; 
this  phenomenon  is  visible  to  the  eye  and  is  rendered 
objective  by  photography. 

Various  other  physicists  and  numbers  of  physi- 
ologists, following  the  path  opened  by  M.  Blondlot, 
published  during  1903  and  1904  manifold  but  often 
rather  hasty  memoirs,  in  which  they  related  the 
results  of  their  researches,  which  do  not  appear 


THE   ETHER  201 

to  have  been  always  conducted  with  the  accuracy 
desirable.  These  results  were  most  strange;  they 
seemed  destined  to  revolutionise  whole  regions  not 
only  of  the  domain  of  physics,  but  likewise  of  the 
biological  sciences.  Unfortunately  the  method  of 
observation  was  always  founded  on  the  variations 
in  visibility  of  the  spark  or  of  a  phosphorescent 
substance,  and  it  soon  became  manifest  that  these 
variations  were  not  perceptible  to  all  eyes. 

No  foreign  experimenter  has  succeeded  in  repeating 
the  experiments,  while  in  France  many  physicists 
have  failed ;  and  hence  the  question  has  much  agi- 
tated public  opinion.  Are  we  face  to  face  with  a 
very  singular  case  of  suggestion,  or  is  special  train- 
ing and  particular  dispositions  required  to  make  the 
phenomenon  apparent  ?  It  is  not  possible,  at  the 
present  moment,  to  declare  the  problem  solved  ;  but 
very  recent  experiments  by  M.  Gut  ton  and  a  note 
by  M.  Mascart  have  reanimated  the  confidence  of 
those  who  hoped  that  such  a  scholar  as  M.  Blondlot 
could  not  have  been  deluded  by  appearances.  How- 
ever, these  last  proofs  in  favour  of  the  existence  of 
the  rays  have  themselves  been  contested,  and  have 
not  succeeded  in  bringing  conviction  to  everyone. 

It  seems  very  probable  indeed  that  certain  of  the 
most  singular  conclusions  arrived  at  by  certain 
authors  on  the  subject  will  lapse  into  deserved 
oblivion.  But  negative  experiments  prove  nothing 
in  a  case  like  this,  and  the  fact  that  most  experi- 


202     THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

menters  have  failed  where  M.  Blondlot  and  his 
pupils  have  succeeded  may  constitute  a  presumption, 
but  cannot  be  regarded  as  a  demonstrative  argument. 
Hence  we  must  still  wait ;  it  is  exceedingly  possible 
that  the  illustrious  physicist  of  Nancy  may  succeed 
in  discovering  objective  actions  of  the  N  rays  which 
shall  be  indisputable,  and  may  thus  establish  on  a 
firm  basis  a  discovery  worthy  of  those  others  which 
have  made  his  name  so  justly  celebrated. 

According  to  M.  Blondlot  the  N  rays  can  be  polar- 
ised, refracted,  and  dispersed,  while  they  have  wave- 
lengths comprised  within  '0030  /m  and  '0760  /u. — that 
is  to  say,  between  an  eighth  and  a  fifth  of  that  found 
for  the  extreme  ultra-violet  rays.  They  might  be, 
perhaps,  simply  rays  of  a  very  short  period.  Their 
existence,  stripped  of  the  parasitical  and  somewhat 
singular  properties  sought  to  be  attributed  to  them, 
would  thus  appear  natural  enough.  It  would,  more- 
over, be  extremely  important,  and  lead,  no  doubt,  to 
most  curious  applications  ;  it  can  be  conceived,  in 
fact,  that  such  rays  might  serve  to  reveal  what 
occurs  in  those  portions  of  matter  whose  too  minute 
dimensions  escape  microscopic  examination  on 
account  of  the  phenomena  of  diffraction. 

From  whatever  point  of  view  we  look  at  it,  and 
whatever  may  be  the  fate  of  the  discovery,  the 
history  of  the  N  rays  is  particularly  instructive,  and 
must  give  food  for  reflection  to  those  interested 
in  questions  of  scientific  methods. 


THE  ETHER  203 

§  6.  THE  ETHER  AND  GRAVITATION 

The  striking  success  of  the  hypothesis  of  the  ether 
in  optics  has,  in  our  own  days,  strengthened  the 
hope  of  being  able  to  explain,  by  an  analogous 
representation,  the  action  of  gravitation. 

For  a  long  time,  philosophers  who  rejected  the 
idea  that  ponderability  is  a  primary  and  essential 
quality  of  all  bodies  have  sought  to  reduce  their 
weight  to  pressures  exercised  in  a  very  subtle  fluid. 
This  was  the  conception  of  Descartes,  and  was 
perhaps  the  true  idea  of  Newton  himself.  Newton 
points  out,  in  many  passages,  that  the  laws  he  had 
discovered  were  independent  of  the  hypotheses  that 
could  be  formed  on  the  way  in  which  universal 
attraction  was  produced,  but  that  with  sufficient 
experiments  the  true  cause  of  this  attraction  might 
one  day  be  reached.  In  the  preface  to  the  second 
edition  of  the  Optics  he  writes :  "  To  prove  that  I 
have  not  considered  weight  as  a  universal  property 
of  bodies,  I  have  added  a  question  as  to  its  cause, 
preferring  this  form  of  question  because  my  interpre- 
tation does  not  entirely  satisfy  me  in  the  absence  of 
experiment "  ;  and  he  puts  the  question  in  this  shape  : 
"  Is  not  this  medium  (the  ether)  more  rarefied  in 
the  interior  of  dense  bodies  like  the  sun,  the  planets, 
the  comets,  than  in  the  empty  spaces  which  separate 
them  ?  Passing  from  these  bodies  to  great  distances, 
does  it  not  become  continually  denser,  and  in  that 


204    THE  NEW   PHYSICS  AND  ITS  EVOLUTION 

way  does  it  not  produce  the  weight  of  these  great 
bodies  with  regard  to  each  other  and  of  their  parts 
with  regard  to  these  bodies,  each  body  tending  to 
leave  the  most  dense  for  the  most  rarefied  parts  ?  " 

Evidently  this  view  is  incomplete,  but  we  may 
endeavour  to  state  it  precisely.  If  we  admit  that 
this  medium,  the  properties  of  which  would  explain 
the  attraction,  is  the  same  as  the  luminous  ether,  we 
may  first  ask  ourselves  whether  the  action  of  gravita- 
tion is  itself  also  due  to  oscillations.  Some  authors 
have  endeavoured  to  found  a  theory  on  this  hypo- 
thesis, but  we  are  immediately  brought  face  to 
face  with  very  serious  difficulties.  Gravity  appears, 
in  fact,  to  present  quite  exceptional  characteristics. 
No  agent,  not  even  those  which  depend  upon  the 
ether,  such  as  light  and  electricity,  has  any  influence 
on  its  action  or  its  direction.  All  bodies  are,  so  to 
speak,  absolutely  transparent  to  universal  attraction, 
and  no  experiment  has  succeeded  in  demonstrating 
that  its  propagation  is  not  instantaneous.  From 
various  astronomical  observations,  Laplace  con- 
cluded that  its  velocity,  in  any  case,  must  exceed 
fifty  million  times  that  of  light.  It  is  subject 
neither  to  reflection  nor  to  refraction;  it  is  in- 
dependent of  the  structure  of  bodies ;  and  not  only 
is  it  inexhaustible,  but  also  (as  is  pointed  out,  accord- 
ing to  M.  Hannequin,  by  an  English  scholar,  James 
Croll)  the  distribution  of  the  effects  of  the  attracting 
force  of  a  mass  over  the  manifold  particles  which 


THE  ETHEE  205 

may  successively  enter  the  field  of  its  action  in  no 
way  diminishes  the  attraction  it  exercises  on  each  of 
them  respectively,  a  thing  which  is  seen  nowhere 
else  in  nature. 

Nevertheless  it  is  possible,  by  means  of  certain 
hypotheses,  to  construct  interpretations  whereby  the 
appropriate  movements  of  an  elastic  medium  should 
explain  the  facts  clearly  enough.  But  these  move- 
ments are  very  complex,  and  it  seems  almost 
inconceivable  that  the  same  medium  could  possess 
simultaneously  the  state  of  movement  corresponding 
to  the  transmission  of  a  luminous  phenomenon  and 
that  constantly  imposed  on  it  by  the  transmission 
of  gravitation. 

Another  celebrated  hypothesis  was  devised  by 
Lesage,  of  Geneva.  Lesage  supposed  space  to  be 
overrun  in  all  directions  by  currents  of  ultra- 
mundane corpuscles.  This  hypothesis,  contested 
by  Maxwell,  is  interesting.  It  might  perhaps  be 
taken  up  again  in  our  days,  and  it  is  not  impos- 
sible that  the  assimilation  of  these  corpuscles  to 
electrons  might  give  a  satisfactory  image.1 

M.  Cremieux  has  recently  undertaken  experiments 
directed,  as  he  thinks,  to  showing  that  the 
divergences  between  the  phenomena  of  gravitation 
and  all  the  other  phenomena  in  nature  are  more 

1  M.  Sagnac  (Le  Radium,  Jan.  1906,  p.  14),  following  perhaps 
Professors  Elster  and  Geitel,  has  lately  taken  up  this  idea 
anew. — ED. 


206    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

apparent  than  real.  Thus  the  evolution  in  the 
heart  of  the  ether  of  a  quantity  of  gravific  energy 
would  not  be  entirely  isolated,  and  as  in  the  case 
of  all  evolutions  of  all  energy  of  whatever  kind,  it 
should  provoke  a  partial  transformation  into  energy 
of  a  different  form.  Thus  again  the  liberated  energy 
of  gravitation  would  vary  when  passing  from  one 
material  to  another,  as  from  gases  into  liquids,  or 
from  one  liquid  to  a  different  one. 

On  this  last  point  the  researches  of  M.  Cremieux 
have  given  affirmative  results:  if  we  immerse  in  a 
large  mass  of  some  liquid  several  drops  of  another 
not  miscible  with  the  first,  but  of  identical  density, 
we  form  a  mass  representing  no  doubt  a  discontinuity 
in  the  ether,  and  we  may  ask  ourselves  whether,  in 
conformity  with  what  happens  in  all  other 
phenomena  of  nature,  this  discontinuity  has  not  a 
tendency  to  disappear. 

If  we  abide  by  the  ordinary  consequences  of 
the  Newtonian  theory  of  potential,  the  drops  should 
remain  motionless,  the  hydrostatic  impulsion  forming 
an  exact  equilibrium  to  their  mutual  attraction. 
Now  M.  Cremieux  remarks  that,  as  a  matter  of 
fact,  they  slowly  approach  each  other. 

Such  experiments  are  very  delicate ;  and  with  all 
the  precautions  taken  by  the  author,  it  cannot  yet 
be  asserted  that  he  has  removed  all  possibility  of 
the  action  of  the  phenomena  of  capillarity  nor  all 
possible  errors  proceeding  from  extremely  slight 


THE  ETHER  207 

differences  of    temperature.      But    the    attempt    is 
interesting  and  deserves  to  be  followed  up. 

Thus,  the  hypothesis  of  the  ether  does  not  yet 
explain  all  the  phenomena  which  the  considerations 
relating  to  matter  are  of  themselves  powerless  to 
interpret.  If  we  wished  to  represent  to  ourselves, 
by  the  mechanical  properties  of  a  medium  filling 
the  whole  of  the  universe,  all  luminous,  electric,  and 
gravitation  phenomena,  we  should  be  led  to  attribute 
to  this  medium  very  strange  and  almost  con- 
tradictory characteristics ;  and  yet  it  would  be 
still  more  inconceivable  that  this  medium  should 
be  double  or  treble,  that  there  should  be  two  or 
three  ethers  each  occupying  space  as  if  it  were  alone, 
and  interpenetrating  it  without  exercising  any  action 
on  one  another.  We  are  thus  brought,  by  a  close 
examination  of  facts,  rather  to  the  idea  that  the 
properties  of  the  ether  are  not  wholly  reducible  to 
the  rules  of  ordinary  mechanics. 

The  physicist  has  therefore  not  yet  succeeded 
in  answering  the  question  often  put  to  him 
by  the  philosopher :  "  Has  the  ether  really  an 
objective  existence  ? "  However,  it  is  not  necessary 
to  know  the  answer  in  order  to  utilize  the  ether. 
In  its  ideal  properties  we  find  the  means  of 
determining  the  form  of  equations  which  are  valid, 
and  to  the  learned  detached  from  all  metaphysical 
prepossession  this  is  the  essential  point. 


CHAPTEE   VII 

A     CHAPTER     IN     THE     HISTORY     OF 
SCIENCE:  WIRELESS  TELEGRAPHY 

§  1 

I  HAVE  endeavoured  in  this  book  to  set  forth  im- 
partially the  ideas  dominant  at  this  moment  in  the 
domain  of  physics,  and  to  make  known  the  facts 
essential  to  them.  I  have  had  to  quote  the  authors 
of  the  principal  discoveries  in  order  to  be  able  to 
class  and,  in  some  sort,  to  name  these  discoveries  ; 
but  I  in  no  way  claim  to  write  even  a  summary 
history  of  the  physics  of  the  day. 

I  am  not  unaware  that,  as  has  often  been  said, 
contemporary  history  is  the  most  difficult  of  all 
histories  to  write.  A  certain  step  backwards  seems 
necessary  in  order  to  enable  us  to  appreciate 
correctly  the  relative  importance  of  events,  and 
details  conceal  the  full  view  from  eyes  which  are 
too  close  to  them,  as  the  trees  prevent  us  from 
seeing  the  forest.  The  event  which  produces  a  great 
sensation  has  often  only  insignificant  consequences ; 

while  another,  which   seemed  at   the  outset  of  the 

208 


WIRELESS   TELEGRAPHY  209 

least  importance  and  little  worthy  of  note,  has  in 
the  long  run  a  widespread  and  deep  influence. 

If,  however,  we  deal  with  the  history  of  a  positive 
discovery,  contemporaries  who  possess  immediate  in- 
formation, and  are  in  a  position  to  collect  authentic 
evidence  at  first  hand,  will  make,  by  bringing  to  it 
their  sincere  testimony,  a  work  of  erudition  which 
may  be  very  useful,  but  which  we  may  be  tempted 
to  look  upon  as  very  easy  of  execution.  Yet  such 
a  labour,  even  when  limited  to  the  study  of  a  very 
minute  question  or  of  a  recent  invention,  is  far  from 
being  accomplished  without  the  historian  stumbling 
over  serious  obstacles. 

An  invention  is  never,  in  reality,  to  be  attributed 
to  a  single  author.  It  is  the  result  of  the  work 
of  many  collaborators  who  sometimes  have  no 
acquaintance  with  one  another,  and  is  often  the 
fruit  of  obscure  labours.  Public  opinion,  however, 
wilfully  simple  in  face  of  a  sensational  discovery, 
insists  that  the  historian  should  also  act  as  judge ; 
and  it  is  the  historian's  task  to  disentangle  the  truth 
in  the  midst  of  the  contest,  and  to  declare  infallibly 
to  whom  the  acknowledgments  of  mankind  should 
be  paid.  He  must,  in  his  capacity  as  skilled  expert, 
expose  piracies,  detect  the  most  carefully  hidden 
plagiarisms,  and  discuss  the  delicate  question  of 
priority;  while  he  must  not  be  deluded  by  those 
who  do  not  fear  to  announce,  in  bold  accents,  that 
they  have  solved  problems  of  which  they  find  the 

H 


210     THE  NEW   PHYSICS  AND  ITS  EVOLUTION 

solution  imminent,  and  who,  the  day  after  its 
'final  elucidation  by  third  parties,  proclaim  them- 
selves its  true  discoverers.  He  must  rise  above  a 
partiality  which  deems  itself  excusable  because  it 
proceeds  from  national  pride ;  and,  finally,  he  must 
seek  with  patience  for  what  has  gone  before. 
While  thus  retreating  step  by  step  he  runs  the 
risk  of  losing  himself  in  the  night  of  time. 

An  example  of  yesterday  seems  to  show  the  diffi- 
culties of  such  a  task.  Among  recent  discoveries 
the  invention  of  wireless  telegraphy  is  one  of  those 
which  have  rapidly  become  popular,  and  looks,  as  it 
were,  an  exact  subject  clearly  marked  out.  Many 
attempts  have  already  been  made  to  write  its  history. 
Mr  J.  J.  Fahie  published  in  England  as  early  as 
1899  an  interesting  work  entitled  the  History  of 
Wireless  Telegraphy ;  and  about  the  same  time 
M.  Broca  published  in  France  a  very  exhaustive 
work  named  La  Telegrapliie  sans  fil.  Among  the 
reports  presented  to  the  Congres  international  de 
physique  (Paris,  1900),  Signor  Righi,  an  illustrious 
Italian  scholar,  whose  personal  efforts  have  largely 
contributed  to  the  invention  of  the  present  system  of 
telegraphy,  devoted  a  chapter,  short,  but  sufficiently 
complete,  of  his  masterly  report  on  Hertzian  waves, 
to  the  history  of  wireless  telegraphy.  The  same 
author,  in  association  with  Herr  Bernhard  Dessau, 
has  likewise  written  a  more  important  work,  Die 
Telegrapliie  ohne  Draht ;  and  La  Telegrapliie  sans  fil 


WIRELESS  TELEGRAPHY  211 

et  les  ondes  filectriques  of  MM.  J.  Boulanger  and  Gr. 
Ferrie  ma}7  also  be  consulted  with  advantage,  as  may 
La  T&fyraphie  sans  fil  of  Signer  Dominico  Mazotto. 
Quite  recently  Mr  A.  Story  has  given  us  in  a  little 
volume  called  The  Story  of  Wireless  Telegraphy,  a 
condensed  but  very  precise  recapitulation  of  all  the 
attempts  which  have  been  made  to  establish  tele- 
graphic communication  without  the  intermediary  of 
a  conducting  wire.  Mr  Story  has  examined  many 
documents,  has  sometimes  brought  curious  facts  to 
light,  and  has  studied  even  the  most  recently 
adopted  apparatus. 

It  may  be  interesting,  by  utilising  the  information 
supplied  by  these  authors  and  supplementing  them 
when  necessary  by  others,  to  trace  the  sources  of 
this  modern  discovery,  to  follow  its  developments, 
and  thus  to  prove  once  more  how  much  a  matter, 
most  simple  in  appearance,  demands  extensive  and 
complex  researches  on  the  part  of  an  author  desirous 
of  writing  a  definitive  work. 

§2 

The  first,  and  not  the  least  difficulty,  is  to  clearly 
define  the  subject.  The  words  "wireless  telegraphy," 
which  at  first  seem  to  correspond  to  a  simple  and 
perfectly  clear  idea,  may  in  reality  apply  to  two 
series  of  questions,  very  different  in  the  mind  of 
a  physicist,  between  which  it  is  important  to 
distinguish. 


212    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

The  transmission  of  signals  demands  three  organs 
which  all  appear  indispensable  :  the  transmitter,  the 
receiver,  and,  between  the  two,  an  intermediary 
establishing  the  communication.  This  intermediary 
is  generally  the  most  costly  part  of  the  installation 
and  the  most  difficult  to  set  up,  while  it  is  here 
that  the  sensible  losses  of  energy  at  the  expense  of 
good  output  occur.  And  yet  our  present  ideas 
cause  us  to  consider  this  intermediary  as  more  than 
ever  impossible  to  suppress  ;  since,  if  we  are 
definitely  quit  of  the  conception  of  action  at  a 
distance,  it  becomes  inconceivable  to  us  that  energy 
can  be  communicated  from  one  point  to  another 
without  being  carried  by  some  intervening  medium. 
But,  practically,  the  line  will  be  suppressed  if, 
instead  of  constructing  it  artificially,  we  use  to 
replace  it  one  of  the  natural  media  which  separate 
two  points  on  the  earth.  These  natural  media  are 
divided  into  two  very  distinct  categories,  and  from 
this  classification  arise  two  series  of  questions  to  be 
examined. 

Between  the  two  points  in  question  there  are,  first, 
the  material  media  such  as  the  air,  the  earth,  and 
the  water.  For  a  long  time  we  have  used  for 
transmissions  to  a  distance  the  elastic  properties  of 
the  air,  and  more  recently  the  electric  conductivity 
of  the  soil  and  of  water,  particularly  that  of  the  sea. 

Modern  physics  leads  us  on  the  other  hand,  as  we 
have  seen,  to  consider  that  there  exists  throughout 


WIRELESS   TELEGRAPHY  213 

the  whole  of  the  universe  another  and  more  subtle 
medium  which  penetrates  everywhere,  is  endowed 
with  elasticity  in  vacuo,  and  retains  its  elasticity 
when  it  penetrates  into  a  great  number  of  bodies, 
such  as  the  air.  This  medium  is  the  luminous  ether 
which  possesses,  as  we  cannot  doubt,  the  property  of 
being  able  to  transmit  energy,  since  it  itself  brings 
to  us  by  far  the  larger  part  of  the  energy  which  we 
possess  on  earth  and  which  we  find  in  the  move- 
ments of  the  atmosphere,  or  of  waterfalls,  and  in 
the  coal  mines  proceeding  from  the  decomposition 
of  carbon  compounds  under  the  influence  of  the 
solar  energy.  For  a  long  time  also  before  the 
existence  of  the  ether  was  known,  the  duty  of 
transmitting  signals  was  entrusted  to  it.  Thus 
through  the  ages  a  double  evolution  is  unfolded 
which  has  to  be  followed  by  the  historian  who  is 
ambitious  of  completeness. 

§  3 

If  such  an  historian  were  to  examine  from  the 
beginning  the  first  order  of  questions,  he  might,  no 
doubt,  speak  only  briefly  of  the  attempts  earlier 
than  electric  telegraphy.  Without  seeking  to  be 
paradoxical,  he  certainly  ought  to  mention  the 
invention  of  the  speaking-trumpet  and  other  similar 
inventions  which  for  a  long  time  have  enabled  man- 
kind, by  the  ingenious  use  of  the  elastic  properties  of 
the  natural  media,  to  communicate  at  greater  distances 


214     THE  NEW   PHYSICS  AND  ITS  EVOLUTION 

than  they  could  have  attained  without  the  aid  of  art. 
After  this  in  some  sort  prehistoric  period  had  been 
rapidly  run  through,  he  would  have  to  follow  very 
closely  the  development  of  electric  telegraphy. 
Almost  from  the  outset,  and  shortly  after  Ampere 
had  made  public  the  idea  of  constructing  a  tele- 
graph, and  the  day  after  Gauss  and  Weber  set  up 
between  their  houses  in  Gottingen  the  first  line 
really  used,  it  was  thought  that  the  conducting 
properties  of  the  earth  and  water  might  be  made  of 
service. 

The  history  of  these  trials  is  very  long,  and  is 
closely  mixed  up  with  the  history  of  ordinary  teleg- 
raphy ;  long  chapters  for  some  time  past  have  been 
devoted  to  it  in  telegraphic  treatises.  It  was  in 
1838,  however,  that  Professor  C.  A.  Steinheil  of 
Munich  expressed,  for  the  first  time,  the  clear  idea 
of  suppressing  the  return  wire  and  replacing  it 
by  a  connection  of  the  line  wire  to  the  earth.  He 
thus  at  one  step  covered  half  the  way,  the  easiest, 
it -is  true,  which  was  to  lead  to  the  final  goal,  since 
he  saved  the  use  of  one-half  of  the  line  of  wire. 
Steinheil,  advised,  perhaps,  by  Gauss,  had,  moreover,  a 
very  exact  conception  of  the  part  taken  by  the  earth 
considered  as  a  conducting  body.  He  seems  to  have 
well  understood  that,  in  certain  conditions,  the 
resistance  of  such  a  conductor,  though  supposed  to 
be  unlimited,  might  be  independent  of  the  distance 
apart  of  the  electrodes  which  carry  'the  current  and 


WIEELESS  TELEGRAPHY  215 

allow  it  to  go  forth.  He  likewise  thought  of  using 
the  railway  lines  to  transmit  telegraphic  signals. 

Several  scholars  who  from  the  first  had  turned 
their  minds  to  telegraphy,  had  analogous  ideas.  It 
was  thus  that  S.  F.  B.  Morse,  superintendent  of  the 
Government  telegraphs  in  the  United  States,  whose 
name  is  universally  known  in  connection  with  the 
very  simple  apparatus  invented  by  him,  made  ex- 
periments in  the  autumn  of  1842  before  a  special 
commission  in  New  York  and  a  numerous  public 
audience,  to  show  how  surely  and  how  easily 
his  apparatus  worked.  In  the  very  midst  of  his 
experiments  a  very  happy  idea  occurred  to  him  of 
replacing  by  the  water  of  a  canal,  the  length  of  about 
a  mile  of  wire  which  had  been  suddenly  and  accident- 
ally destroyed.  This  accident,  which  for  a  moment 
compromised  the  legitimate  success  the  celebrated 
engineer  expected,  thus  suggested  to  him  a  fruitful 
idea  which  he  did  not  forget.  He  subsequently 
repeated  attempts  to  thus  utilise  .the  earth  and 
water,  and  obtained  some  very  remarkable  results. 

It  is  not  possible  to  quote  here  all  the  researches 
undertaken  with  the  same  purpose,  to  which  are 
more  particularly  attached  the  names  of  S.  W. 
Wilkins,  Wheatstone,  and  H.  Highton,  in  England  ; 
of  Bonetti  in  Italy,  Gintl  in  Austria,  Bouchot  and 
Donat  in  France  ;  but  there  are  some  which  cannot 
be  recalled  without  emotion. 

On  the  17th  December  1870,  a  physicist  who  has 


2i6    THE  NEW   PHYSIOS  AND   ITS  EVOLUTION 

left  in  the  University  of  Paris  a  lasting  name,  M. 
d'Almeida^at  that  time  Professor  at  the  Lycee  Henri 
IV.  and  later  Inspector-General  of  Public  Instruc- 
tion, quitted  Paris,  then  besieged,  in  a  balloon,  and 
descended  in  the  midst  of  the  German  lines.  He 
succeeded,  after  a  perilous  journey,  in  gaming  Havre 
by  way  of  Bordeaux  and  Lyons ;  and  after  procuring 
the  necessary  apparatus  in  England,  he  descended 
the  Seine  as  far  as  Poissy,  which  he  reached  on  the 
14bh  January  1871.  After  his  departure,  two  other 
scholars,  MM.  Desains  and  Bourbouze,  relieving 
each  other  day  and  night,  waited  at  Paris,  in  a 
wherry  on  the  Seine,  ready  to  receive  the  signal 
which  they  awaited  with  patriotic  anxiety.  It  was 
a  question  of  working  a  process  devised  by  the 
last-named  pair,  in  which  the  water  of  the  river 
acted  the  part  of  the  line  wire.  On  the  23rd 
January  the  communication  at  last  seemed  to  be 
established,  but  unfortunately,  first  the  armistice 
and  then  the  surrender  of  Paris  rendered  useless 
the  valuable  result  of  this  noble  effort. 

Special  mention  is  also  due  to  the  experiments 
made  by  the  Indian  Telegraph  Office,  under  the 
direction  of  Mr  Johnson  and  afterwards  of  Mr  W.  F. 
Melhuish.  They  led,  indeed,  in  1889  to  such  satis- 
factory results  that  a  telegraph  service,  in  which  the 
line  wire  was  replaced  by  the  earth,  worked  practi- 
cally and  regularly.  Other  attempts  were  also  made 
during  the  latter  half  of  the  nineteenth  century  to 


WIRELESS  TELEGRAPHY  217 

transmit  signals  through  the  sea.  They  preceded 
the  epoch  when,  thanks  to  numerous  physicists, 
among  whom  Lord  Kelvin  undoubtedly  occupies  a 
preponderating  position,  we  succeeded  in  sinking 
the  first  cable ;  but  they  were  not  abandoned,  even 
after  that  date,  for  they  gave  hopes  of  a  much  more 
economical  solution  of  the  problem.  Among  the 
most  interesting  are  remembered  those  that  S.  W. 
Wilkins  carried  on  for  a  long  time  between  France 
and  England.  Like  Cooke  and  Wheatstone,  he 
thought  of  using  as  a  receiver  an  apparatus  which 
in  some  features  resembles  the  present  receiver  of 
the  submarine  telegraph.  Later,  George  E.  Bering, 
then  James  Bowman  and  Lindsay,  made  on  the  same 
lines  trials  which  are  worthy  of  being  remembered. 

But  it  is  only  in  our  own  days  that  Sir  William 
H.  Preece  at  last  obtained  for  the  first  time  really 
practical  results.  Sir  William  himself  effected  and 
caused  to  be  executed  by  his  associates — he  is  chief 
consulting  engineer  to  the  General  Post  Office  in 
England — researches  conducted  with  much  method 
and  based  on  precise  theoretical  considerations.  He 
thus  succeeded  in  establishing  very  easy,  clear,  and 
regular  communications  between  various  places ;  for 
example,  across  the  Bristol  Channel.  The  long 
series  of  operations  accomplished  by  so  many 
seekers,  with  the  object  of  substituting  a  material 
and  natural  medium  for  the  artificial  lines  of  metal, 
thus  met  with  an  undoubted  success  which  was  soon 


218     THE  NEW  PHYSICS   AND   ITS  EVOLUTION 

to   be   eclipsed   by  the   widely-known   experiments 
directed  into  a  different  line  by  Marconi. 

It  is  right  to  add  that  Sir  William  Preece  had 
himself  utilised  induction  phenomena  in  his  ex- 
periments, and  had  begun  researches  with  the  aid 
of  electric  waves.  Much  is  due  to  him  for  the 
welcome  he  gave  to  Marconi ;  it  is  certainly  thanks 
to  the  advice  and  the  material  support  he  found 
in  Sir  William  that  the  young  scholar  succeeded  in 
effecting  his  sensational  experiments. 

§4 

The  starting-point  of  the  experiments  based  on 
the  properties  of  the  luminous  ether,  and  having  for 
their  object  the  transmission  of  signals,  is  very 
remote ;  and  it  would  be  a  very  laborious  task  to  hunt 
up  all  the  work  accomplished  in  that  direction,  even 
if  we  were  to  confine  ourselves  to  those  in  which 
electrical  reactions  play  a  part.  An  electric  reaction, 
an  electrostatic  influence,  or  an  electromagnetic 
phenomenon,  is  transmitted  at  a  distance  through  the 
air  by  the  intermediary  of  the  luminous  ether.  But 
electric  influence  can  hardly  be  used,  as  the  distances 
it  would  allow  us  to  traverse  would  be  much  too 
restricted,  and  electrostatic  actions  are  often  very 
erratic.  The  phenomena  of  induction,  which  are  very 
regular  and  insensible  to  the  variations  of  the  atmo- 
sphere, have,  on  the  other  hand,  for  a  long  time 
appeared  serviceable  for  telegraphic  purposes. 


WIRELESS   TELEGRAPHY  219 

We  might  find,  in  a  certain  number  of  the 
attempts  just  mentioned,  a  partial  employment  of 
these  phenomena.  Lindsay,  for  instance,  in  his 
project  of  communication  across  the  sea,  attributed 
to  them  a  considerable  rdle.  These  phenomena  even 
permitted  a  true  telegraphy  without  intermediary 
wire  between  the  transmitter  and  the  receiver,  at  very 
restricted  distances,  it  is  true,  but  in  peculiarly 
interesting  conditions.  It  is,  in  fact,  owing  to 
them  that  C.  Brown,  and  later  Edison  and  Gilliland, 
succeeded  in  establishing  communications  with  trains 
in  motion. 

Mr  Willoughby  S.  Smith  and  Mr  Charles  A. 
Stevenson  also  undertook  experiments  during  the 
last  twenty  years,  in  which  they  used  induction, 
but  the  most  remarkable  attempts  are  perhaps 
those  of  Professor  Emile  Eathenau.  With  the 
assistance  of  Professor  Eubens  and  of  Herr  W. 
Eathenau,  this  physicist  effected,  at  the  request 
of  the  German  Ministry  of  Marine,  a  series  of 
researches  which  enabled  him,  by  means  of  a  com- 
pound system  of  conduction  and  induction  by  alter- 
nating currents,  to  obtain  clear  and  regular  com- 
munications at  a  distance  of  four  kilometres. 
Among  the  precursors  also  should  be  mentioned 
Graham  Bell ;  the  inventor  of  the  telephone  thought 
of  employing  his  admirable  apparatus  as  a  receiver 
of  induction  phenomena  transmitted  from  a  dis- 
tance ;  Edison,  Herr  Sacher  of  Vienna,  M.  Henry 


220    THE  NEW   PHYSICS  AND   ITS  EVOLUTION 

Dufour  of  Lausanne,  and  Professor  Trowbridge  of 
Boston,  also  made  interesting  attempts  in  the  same 
direction. 

In  all  these  experiments  occurs  the  idea  of  em- 
ploying an  oscillating  current.  Moreover,  it  was 
known  for  a  long  time — since,  in  1842,  the  great 
American  physicist  Henry  proved  that  the  dis- 
charges from  a  Leyden  jar  in  the  attic  of  his 
house  caused  sparks  in  a  metallic  circuit  on  the 
ground  floor — that  a  flux  which  varies  rapidly  and 
periodically  is  much  more  efficacious  than  a  simple 
flux,  which  latter  can  only  produce  at  a  distance  a 
phenomenon  of  slight  intensity.  This  idea  of  the 
oscillating  current  was  closely  akin  to  that  which 
was  at  last  to  lead  to  an  entirely  satisfactory 
solution :  that  is,  to  a  solution  which  is  founded  on 
the  properties  of  electric  waves. 

§5 

Having  thus  got  to  the  threshold  of  -the  definitive 
edifice,  the  historian,  who  has  conducted  his  readers 
over  the  two  parallel  routes  which  have  just  been 
marked  out,  will  be  brought  to  ask  himself  whether 
he  has  been  a  sufficiently  faithful  guide  and  has  not 
omitted  to  draw  attention  to  all  essential  points  in 
the  regions  passed  through. 

Ought  we  not  to  place  by  the  side,  or  perhaps  in 
front,  of  the  authors  who  have  devised  the  practical 
appliances,  those  scholars  who  have  constructed  the 


WIRELESS  TELEGRAPHY  221 

theories  and  realised  the  laboratory  experiments  of 
which,  after  all,  the  apparatus  are  only  the  immedi- 
ate applications  ?  If  we  speak  of  the  propagation 
of  a  current  in  a  material  medium,  can  one  forget 
the  names  of  Fourier  and  of  Ohm,  who  established 
by  theoretical  considerations  the  laws  which  preside 
over  this  propagation  ?  When  one  looks  at  the 
phenomena  of  induction,  would  it  not  be  just  to 
remember  that  Arago  foresaw  them,  and  that  Michael 
Faraday  discovered  them  ?  It  would  be  a  delicate, 
and  also  a  rather  puerile  task,  to  class  men  of  genius 
in  order  of  merit.  The  merit  of  an  inventor  like 
Edison  and  that  of  a  theorist  like  Clerk  Maxwell 
have  no  common  measure,  and  mankind  is  indebted 
for  its  great  progress  to  the  one  as  much  as  to 
the  other. 

Before  relating  how  success  attended  the  efforts 
to  utilise  electric  waves  for  the  transmission  of 
signals,  we  cannot  without  ingratitude  pass  over 
in  silence  the  theoretical  speculations  and  the  work 
of  pure  science  which  led  to  the  knowledge  of  these 
waves.  It  would  therefore  be  just,  without  going 
further  back  than  Faraday,  to  say  how  that  illustrious 
physicist  drew  attention  to  the  part  taken  by  in- 
sulating media  in  electrical  phenomena,  and  to  insist 
also  on  the  admirable  memoirs  in  which  for  the 
first  time  Clerk  Maxwell  made  a  solid  bridge 
between  those  two  great  chapters  of  Physics,  optics 
and  electricity,  which  till  then  had  been  inde- 


222    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

pendent  of  each  other.  And  no  doubt  it  would  be 
impossible  not  to  evoke  the  memory  of  those  who, 
by  establishing,  -on  the  other  hand,  the  solid  and 
magnificent  structure  of  physical  optics,  and  proving 
by  their  immortal  works  the  undulatory  nature  of 
light,  prepared  from  the  opposite  direction  the  future 
unity.  In  the  history  of  the  applications  of  electrical 
undulations,  the  names  of  Young,  Fresnel,  Fizeau, 
and  Foucault  must  be  inscribed  ;  without  these 
scholars,  the  assimilation  between  electrical  and 
luminous  phenomena  which  they  discovered  and 
studied  would  evidently  have  been  impossible. 

Since  there  is  an  absolute  identity  of  nature 
between  the  electric  and  the  luminous  waves,  we 
should,  in  all  justice,  also  consider  as  precursors 
those  who  devised  the  first  luminous  telegraphs. 
Claude  Chappe  incontestably  effected  wireless  teleg- 
raphy, thanks  to  the  luminous  ether,  and  the 
learned  men,  such  as  Colonel  Mangin,  who  perfected 
optical  telegraphy,  indirectly  suggested  certain  im- 
provements lately  introduced  into  the  present 
method. 

But  the  physicist  whose  work  should  most  of  all 
be  put  in  evidence  is,  without  fear  of  contradic- 
tion, Heinrich  Hertz.  It  was  he  who  demonstrated 
irrefutably,  by  experiments  now  classic,  that  an 
electric  discharge  produces  an  undulatory  disturb- 
ance in  the  ether  contained  in  the  insulating  media 
in  its  neighbourhood ;  it  was  he  who,  as  a  profound 


WIRELESS  TELEGRAPHY  223 

theorist,  a  clever  mathematician,  and  an  experimenter 
of  prodigious  dexterity,  made  known  the  mechanism 
of  the  production,  and  fully  elucidated  that  of  the 
propagation  of  these  electromagnetic  waves. 

He  must  naturally  himself  have  thought  that 
his  discoveries  might  be  applied  to  the  transmission 
of  signals.  It  would  appear,  however,  that  when 
interrogated  by  a  Munich  engineer  named  Huber 
as  to  the  possibility  of  utilising  the  waves  for  trans- 
missions by  telephone,  he  answered  in  the  negative, 
and  dwelt  on  certain  considerations  relative  to  the 
difference  between  the  periods  of  sounds  and  those 
of  electrical  vibrations.  This  answer  does  not 
allow  us  to  judge  what  might  have  happened,  had 
not  a  cruel  death  carried  off  in  1894,  at  the  age  of 
thirty- five,  the  great  and  unfortunate  physicist. 

We  might  also  find  in  certain  works  earlier  than 
the  experiments  of  Hertz  attempts  at  transmission 
in  which,  unconsciously  no  doubt,  phenomena  were 
already  set  in  operation  which  would,  at  this  day, 
be  classed  as  electric  oscillations.  It  is  allow- 
able no  doubt,  not  to  speak  of  an  American 
quack,  Mahlon  Loomis,  who,  according  to  Mr  Story, 
patented  in  1870  a  project  of  communication  in 
which  he  utilised  the  Eocky  Mountains  on  one  side 
and  Mont  Blanc  on  the  other,  as  gigantic  antennse 
to  establish  communication  across  the  Atlantic ;  but 
we  cannot  pass  over  in  silence  the  very  remarkable 
researches  of  the  American  Professor  Dolbear,  who 


224    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

showed,  at  the  electrical  exhibition  of  Philadelphia 
in  1884,  a  set  of  apparatus  enabling  signals  to  be 
transmitted  at  a  distance,  which  he  described  as 
"an  exceptional  application  of  the  principles  of 
electrostatic  induction."  This  apparatus  comprised 
groups  of  coils  and  condensers  by  means  of  which 
he  obtained,  as  we  cannot  now  doubt,  effects  due  to 
true  electric  waves. 

Place  should  also  be  made  for  a  well-known 
inventor,  D.  E.  Hughes,  who  from  1879  to  1886 
followed  up  some  very  curious  experiments  in 
which  also  these  oscillations  certainly  played  a  con- 
siderable part.  It  was  this  physicist  who  invented 
the  microphone,  and  thus,  in  another  way,  drew 
attention  to  the  variations  of  contact  resistance,  a 
phenomenon  not  far  from  that  produced  in  the  radio- 
conductors  of  Branly,  which  are  important  organs  in 
the  Marconi  system.  Unfortunately,  fatigued  and 
in  ill-health,  Hughes  ceased  his  researches  at  the 
moment  perhaps  when  they  would  have  given  him 
final  results. 

In  an  order  of  ideas  different  in  appearance,  but 
closely  linked  at  bottom  with  the  one  just  men- 
tioned, must  be  recalled  the  discovery  of  radiophony 
in  1880  by  Graham  Bell,  which  was  foreshadowed  in 
1875  by  C.  A.  Brown.  A  luminous  ray  falling  on 
a  selenium  cell  produces  a  variation  of  electric 
resistance,  thanks  to  which  a  sound  signal  can  be 
transmitted  by  light.  That  delicate  instrument  the 


WIRELESS  TELEGRAPHY  225 

radiophone,  constructed  on  this  principle,  has  wide 
analogies  with  the  apparatus  of  to-day. 

§6 

Starting  from  the  experiments  of  Hertz,  the 
history  of  wireless  telegraphy  almost  merges  into 
that  of  the  researches  on  electrical  waves.  All 
the  progress  realised  in  the  manner  of  producing 
and  receiving  these  waves  necessarily  helped  to  give 
rise  to  the  application  already  indicated.  The  ex- 
periments of  Hertz,  after  being  checked  in  every 
laboratory,  and  having  entered  into  the  strong 
domain  of  our  most  certain  knowledge,  were  about 
to  yield  the  expected  fruit. 

Experimenters  like  Sir  Oliver  Lodge  in  England, 
Righi  in  Italy,  Sarrazin  and  de  la  Rive  in  Switzer- 
land, Blondlot  in  France,  Lecher  in  Germany,  Bose 
in  India,  Lebedeff  in  Russia,  and  theorists  like  M.  H. 
Poincare  and  Professor  Bjerknes,  who  devised  in- 
genious arrangements  or  elucidated  certain  points 
left  dark,  are  among  the  artisans  of  the  work  which 
followed  its  natural  evolution. 

It  was  Professor  R.  Threlfall  who  seems  to  have 
been  the  first  to  clearly  propose,  in  1890,  the  applica- 
tion of  the  Hertzian  waves  to  telegraphy,  but  it  was 
certainly  Sir  W.  Crookes  who,  in  a  very  remarkable 
article  in  the  Fortnightly  Review  of  February  1892, 
pointed  out  very  clearly  the  road  to  be  followed. 
He  even  showed  in  what  conditions  the  Morse 


226    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

receiver  might  be  applied  to  the  new  system  of 
telegraphy. 

About  the  same  period  an  American  physicist, 
well  known  by  his  celebrated  experiments  on  high 
frequency  currents — experiments,  too,  which  are  not 
unconnected  with  those  on  electric  oscillations, — 
M.  Tesla,  demonstrated  that  these  oscillations  could 
be  transmitted  to  more  considerable  distances  by 
making  use  of  two  vertical  antennae,  terminated  by 
large  conductors. 

A  little  later,  Sir  Oliver  Lodge  succeeded,  by 
the  aid  of  the  coherer,  in  detecting  waves  at 
relatively  long  distances,  and  Mr  Rutherford  obtained 
similar  results  with  a  magnetic  indicator  of  his  own 
invention. 

An  important  question  of  meteorology,  the  study 
of  atmospheric  discharges,  at  this  date  led  a  few 
scholars,  and  more  particularly  the  Russian,  M. 
Popoff,  to  set  up  apparatus  very  analogous  to  the 
receiving  apparatus  of  the  present  wireless  teleg- 
raphy. This  comprised  a  long  antenna  and  filings- 
tube,  and  M.  Popoff  even  pointed  out  that  his 
apparatus  might  well  serve  for  the  transmission  of 
signals  as  soon  as  a  generator  of  waves  powerful 
enough  had  been  discovered. 

Finally,  on  the  2nd  June  1896,  a  young  Italian, 
born  in  Bologna  on  the  25th  April  1874,  Guglielnio 
Marconi,  patented  a  system  of  wireless  telegraphy 
destined  to  become  rapidly  popular.  Brought  up 


WIRELESS   TELEGRAPHY  227 

in  the  laboratory  of  Professor  Righi,  one  of  the 
physicists  who  had  done  most  to  confirm  and  extend 
the  experiments  of  Hertz,  Marconi  had  long  been 
familiar  with  the  properties  of  electric  waves,  and 
was  well  used  to  their  manipulation.  He  after- 
wards had  the  good  fortune  to  meet  Sir  William 
(then  Mr)  Preece,  who  was  to  him  an  adviser  of  the 
highest  authority. 

It  has  sometimes  been  said  that  the  Marconi 
system  contains  nothing  original ;  that  the  apparatus 
for  producing  the  waves  was  the  oscillator  of  Righi, 
that  the  receiver  was  that  employed  for  some  two  or 
three  years  by  Professor  Lodge  and  Mr  Bose,  and  was 
founded  on  an  earlier  discovery  by  a  French  scholar, 
M.  Branly ;  and,  finally,  that  the  general  arrangement 
was  that  established  by  M.  Popoff. 

The  persons  who  thus  rather  summarily  judge 
the  work  of  M.  Marconi  show  a  severity  approaching 
injustice.  It  cannot,  in  truth,  be  denied  that  the 
young  scholar  has  brought  a  strictly  personal  contri- 
bution to  the  solution  of  the  problem  he  proposed 
to  himself.  Apart  from  his  forerunners,  and  when 
their  attempts  were  almost  unknown,  he  had  the 
very  great  merit  of  adroitly  arranging  the  most 
favourable  combination,  and  he  was  the  first  to 
succeed  in  obtaining  practical  results,  while  he 
showed  that  the  electric  waves  could  be  transmitted 
and  received  at  distances  enormous  compared  to 
those  attained  before  his  day. 


228    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

Alluding  to  a  well-known  anecdote  relating  to 
Christopher  Columbus,  Sir  W.  Preece  very  justly 
said :  "  The  forerunners  and  rivals  of  Marconi  no 
doubt  knew  of  the  eggs,  but  he  it  was  who  taught 
them  to  make  them  stand  on  end."  This  judgment 
will,  without  any  doubt,  be  the  one  that  history  will 
definitely  pronounce  on  the  Italian  scholar. 

§7 

The  apparatus  which  enables  the  electric  waves 
to  be  revealed,  the  detector  or  indicator,  is  the 
most  delicate  organ  in  wireless  telegraphy.  It  is 
not  necessary  to  employ  as  an  indicator  a  filings-tube 
or  radio-conductor.  One  can,  in  principle,  for  the 
purpose  of  constructing  a  receiver,  think  of  any 
one  of  the  multiple  effects  produced  by  the  Hertzian 
waves.  In  many  systems  in  use,  and  in  the  new 
one  of  Marconi  himself,  the  use  of  these  tubes  has 
been  abandoned  and  replaced  by  magnetic  detectors. 

Nevertheless,  the  first  and  the  still  most  frequent 
successes  are  due  to  radio-conductors,  and  public 
opinion  has  not  erred  in  attributing  to  the  inventor 
of  this  ingenious  apparatus  a  considerable  and 
almost  preponderant  part  in  the  invention  of  wave 
telegraphy. 

The  history  of  the  discovery  of  radio-conductors 
is  short,  but  it  deserves,  from  its  importance,  a 
chapter  to  itself  in  the  history  of  wireless  telegraphy. 
From  a  theoretical  point  of  view,  the  phenomena 


WIRELESS  TELEGRAPHY  229 

produced  in  those  tubes  should  be  set  by  the  side 
of  those  studied  by  Graham  Bell,  C.  A.  Brown,  and 
Summer  Tainter,  from  the  year  1878  onward.  The 
variations  to  which  luminous  waves  give  rise  in  the 
resistance  of  selenium  and  other  substances  are, 
doubtless,  not  unconnected  with  those  which  the 
electric  waves  produce  in  filings.  A  connection  can 
also  be  established  between  this  effect  of  the  waves 
and  the  variations  of  contact  resistance  which  enabled 
Hughes  to  construct  the  microphone,  that  admirable 
instrument  which  is  one  of  the  essential  organs  of 
telephony. 

More  directly,  as  an  antecedent  to  the  discovery, 
should  be  quoted  the  remark  made  by  Varley  in 
1870,  that  coal-dust  changes  in  conductivity  when 
the  electromotive  force  of  the  current  which  passes 
through  it  is  made  to  vary.  But  it  was  in  1884 
that  an  Italian  professor,  Signor  Calzecchi-Onesti, 
demonstrated  in  a  series  of  remarkable  experiments 
that  the  metallic  filings  contained  in  a  tube  of  in- 
sulating material,  into  which  two  metallic  electrodes 
are  inserted,  acquire  a  notable  conductivity  under 
different  influences  such  as  extra  currents,  induced 
currents,  sonorous  vibrations,  etc.,  and  that  this  con- 
ductivity is  easily  destroyed ;  as,  for  instance,  by 
turning  the  tube  over  and  over. 

In  several  memoirs  published  in  1890  and  1891, 
M.  Ed.  Branly  independently  pointed  out  similar 
phenomena,  and  made  a  much  more  complete  and 


230    THE  NEW  PHYSICS   AND   ITS   EVOLUTION 

systematic  study  of  the  question.  He  was  the  first 
to  note  very  clearly  that  the  action  described  could 
be  obtained  by  simply  making  sparks  pass  in  the 
neighbourhood  of  the  radio-conductor,  and  that  their 
great  resistance  could  be  restored  to  the  filings  by 
giving  a  slight  shake  to  the  tube  or  to  its  supports. 

The  idea  of  utilising  such  a  very  interesting 
phenomenon  as  an  indicator  in  the  study  of  the 
Hertzian  waves  seems  to  have  occurred  simul- 
taneously to  several  physicists,  among  whom  should 
be  especially  mentioned  M.  Ed.  Branly  himself,  Sir 
Oliver  Lodge,  and  MM.  Le  Koyer  and  Van  Beschem, 
and  its  use  in  laboratories  rapidly  became  quite 
common. 

The  action  of  the  waves  on  metallic  powders  has, 
ho wever,remained somewhat  mysterious;  for  ten  years 
it  has  been  the  subject  of  important  researches  by 
Professor  Lodge,  M.  Branly,  and  a  very  great  number 
of  the  most  distinguished  physicists.  It  is  impossible 
to  notice  here  all  these  researches,  but  from  a  recent 
and  very  interesting  work  of  M.  Blanc,  it  would  seem 
that  the  phenomenon  is  allied  to  that  of  ionisation. 

§8 

The  history  of  wireless  telegraphy  does  not  end 
with  the  first  experiments  of  Marconi;  but  from 
the  moment  their  success  was  announced  in  the 
public  press,  the  question  left  the  domain  of  pure 
science  to  enter  into  that  of  commerce. 


WIRELESS  TELEGRAPHY  231 

The  historian's  task  here  becomes  different,  but 
even  more  delicate  ;  and  he  will  encounter  difficulties 
which  can  be  only  known  to  one  about  to  write  the 
history  of  a  commercial  invention. 

The  actual  improvements  effected  in  the  system 
are  kept  secret  by  the  rival  companies,  and  the 
most  important  results  are  patriotically  left  in  dark- 
ness by  the  learned  officers  who  operate  discreetly 
in  view  of  the  national  defence.  Meanwhile,  men 
of  business  desirous  of  bringing  out  a  company  pro- 
claim, with  great  nourish  of  advertisements,  that 
they  are  about  to  exploit  a  process  superior  to  all 
others. 

On  this  slippery  ground  the  .impartial  historian 
must  nevertheless  venture ;  and  he  may  not  refuse 
to  relate  the  progress  accomplished,  which  is  con- 
siderable. Therefore,  after  having  described  the 
experiments  carried  out  for  nearly  ten  years  by 
Marconi  himself,  first  across  the  Bristol  Channel, 
then  at  Spezzia,  between  the  coast  and  the  iron- 
clad San  Bartolommeo,  and  finally  by  means  of 
gigantic  apparatus  between  America  and  England, 
he  must  give  the  names  of  those  who,  in  the 
different  civilised  countries,  have  contributed  to 
the  improvement  of  the  system  of  communication 
by  waves;  while  he  must  describe  what  precious 
services  this  system  has  already  rendered  to  the 
art  of  war,  and  happily  also  to  peaceful  navigation. 

From  the   point   of  view   of   the   theory   of  the 


232    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

phenomena,  very  remarkable  results  have  been 
obtained  by  various  physicists,  among  whom  should 
be  particularly  mentioned  M.  Tissot,  whose  bril- 
liant studies  have  thrown  a  bright  light  on 
different  interesting  points,  such  as  the  role  of  the 
antennae.  It  would  be  equally  impossible  to  pass 
over  in  silence  other  recent  attempts  in  a  slightly 
different  groove.  Marconi's  system,  however  im- 
proved it  may  be  to-day,  has  one  grave  defect. 
The  synchronism  of  the  two  pieces  of  apparatus, 
the  transmitter  and  the  receiver,  is  not  perfect,  so 
that  a  message  sent  off  by  one  station  may  be 
captured  by  some  other  station.  The  fact  that  the 
phenomena  of  resonance  are  not  utilised,  further 
prevents  the  quantity  of  energy  received  by  the 
receiver  from  being  considerable,  and  hence  the 
effects  reaped  are  very  weak,  so  that  the  system 
remains  somewhat  fitful  and  the  communications 
are  often  disturbed  by  atmospheric  phenomena. 
Causes  which  render  the  air  a  momentary  conductor, 
such  as  electrical  discharges,  ionisation,  etc.,  moreover 
naturally  prevent  the  waves  from  passing,  the  ether 
thus  losing  its  elasticity. 

Professor  Ferdinand  Braun  of  Strasburg  has 
conceived  the  idea  of  employing  a  mixed  system, 
in  which  the  earth  and  the  water,  which,  as  we 
have  seen,  have  often  been  utilised  to  conduct  a 
current  for  transmitting  a  signal,  will  serve  as  a 
sort  of  guide  to  the  waves  themselves.  The  now 


WIRELESS   TELEGRAPHY  233 

well-known  theory  of  the  propagation  of  waves 
guided  by  a  conductor  enables  it  to  be  foreseen 
that,  according  to  their  periods,  these  waves  will 
penetrate  more  or  less  deeply  into  the  natural 
medium,  from  which  fact  has  been  devised  a  method 
of  separating  them  according  to  their  frequency.  By 
applying  this  theory,  M.  Braun  has  carried  out,  first 
in  the  fortifications  of  Strasburg,  and  then  between 
the  island  of  Heligoland  and  the  mainland,  experi- 
ments which  have  given  remarkable  results.  We 
might  mention  also  the  researches,  in  a  somewhat 
analogous  order  of  ideas,  by  an  English  engineer, 
Mr  Armstrong,  by  Dr  Lee  de  Forest,  and  also  by 
Professor  Fessenden. 

Having  thus  arrived  at  the  end  of  this  long 
journey,  which  has  taken  him  from  the  first  attempts 
down  to  the  most  recent  experiments,  the  historian 
can  yet  set  up  no  other  claim  but  that  of 
having  written  the  commencement  of  a  history 
which  others  must  continue  in  the  future.  Progress 
does  not  stop,  and  it  is  never  permissible  to  say  that 
an  invention  has  reached  its  final  form. 

Should  the  historian  desire  to  give  a  conclusion  to 
his  labour  and  answer  the  question  the  reader  would 
doubtless  not  fail  to  put  to  him,  "To  whom,  in 
short,  should  the  invention  of  wireless  telegraphy 
more  particularly  be  attributed  ? "  he  should 
certainly  first  give  the  name  of  Hertz,  the  genius 
who  discovered  the  waves,  then  that  of  Marconi, 


234    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

who  was  the  first  to  transmit  signals  by  the  use  of 
Hertzian  undulations,  and  should  add  those  of  the 
scholars  who,  like  Morse,  Popoff,  Sir  W.  Preece,  Lodge, 
and,  above  all,  Branly,  have  devised  the  arrangements 
necessary  for  their  transmission.  But  he  might 
then  recall  what  Voltaire  wrote  in  the  Philosophical 
Dictionary : 

"  What !  We  wish  to  know  what  was  the  exact 
theology  of  Thot,  of  Zerdust,  of  Sanchuniathon,  of 
the  first  Brahmins,  and  we  are  ignorant  of  the 
inventor  of  the  shuttle !  The  first  weaver,  the  first 
mason,  the  first  smith,  were  no  doubt  great  geniuses, 
but  they  were  disregarded.  Why  ?  Because  none 
of  them  invented  a  perfected  art.  The  one  who 
hollowed  out  an  oak  to  cross  a  river  never  made  a 
galley ;  those  who  piled  up  rough  stones  with  girders 
of  wood  did  not  plan  the  Pyramids.  Everything  is 
made  by  degrees  and  the  glory  belongs  to  no  one." 

To-day,  more  than  ever,  the  words  of  Voltaire  are 
true:  science  becomes  more  and  more  impersonal, 
and  she  teaches  us  that  progress  is  nearly  always  due 
to  the  united  efforts  of  a  crowd  of  workers,  and  is 
thus  the  best  school  of  social  solidarity. 


CHAPTEK  VIII 

THE  CONDUCTIVITY  OF  GASES 
AND  THE   IONS 

§  1.  THE  CONDUCTIVITY  OF  GASES 

IF  we  were  confined  to  the  facts  I  have  set  forth 
above,  we  might  conclude  that  two  classes  of  pheno- 
mena are  to-day  being  interpreted  with  increasing 
correctness  in  spite  of  the  few  difficulties  which  have 
been  pointed  out.  The  hypothesis  of  the  molecular 
constitution  of  matter  enables  us  to  group  together 
one  of  these  classes,  and  the  hypothesis  of  the  ether 
leads  us  to  co-ordinate  the  other. 

But  these  two  classes  of  phenomena  cannot  be 
considered  independent  of  each  other.  Kelations 
evidently  exist  between  matter  and  the  ether,  which 
manifest  themselves  in  many  cases  accessible  to 
experiment,  and  the  search  for  these  relations 
appears  to  be  the  paramount  problem  the  physicist 
should  set  himself.  The  question  has,  for  a  long 
time,  been  attacked  on  various  sides,  but  the  recent 
discoveries  in  the  conductivity  of  gases,  of  the  radio- 
active substances,  and  of  the  cathode  and  similar 

235 


236     THE   NEW   PHYSICS  AND  ITS  EVOLUTION 

rays,  have  allowed  us  of  late  years  to  regard  it  in 
a  new  light.  Without  wishing  to  set  out  here  in 
detail  facts  which  for  the  most  part  are  well  known, 
we  will  endeavour  to  group  the  chief  of  them  round 
a  few  essential  ideas,  and  will  seek  to  state  precisely 
the  data  they  afford  us  for  the  solution  of  this  grave 
problem. 

It  was  the  study  of  the  conductivity  of  gases 
which  at  the  very  first  furnished  the  most  important 
information,  and  allowed  us  to  penetrate  more 
deeply  than  had  till  then  been  possible  into  the 
inmost  constitution  of  matter,  and  thus  to,  as  it 
were,  catch  in  the  act  the  actions  that  matter  can 
exercise  on  the  ether,  or,  reciprocally,  those  it  may 
receive  from  it. 

It  might,  perhaps,  have  been  foreseen  that  such  a 
study  would  prove  remarkably  fruitful.  The  exa- 
mination of  the  phenomena  of  electrolysis  had,  in 
fact,  led  to  results  of  the  highest  importance  on  the 
constitution  of  liquids,  and  the  gaseous  media  which 
presented  themselves  as  particularly  simple  in  all 
their  properties  ought,  it  would  seem,  to  have 
supplied  from  the  very  first  a  field  of  investigation 
easy  to  work  and  highly  productive. 

This,  however,  was  not  at  all  the  case.  Experi- 
mental complications  springing  up  at  every  step 
obscured  the  problem.  One  generally  found  one's 
self  in  the  presence  of  violent  disruptive  discharges 
with  a  train  of  accessory  phenomena,  due,  for  in- 


CONDUCTIVITY  OF  GASES  AND   THE  IONS    237 

stance,  to  the  use  of  metallic  electrodes,  and  made 
evident  by  the  complex  appearance  of  aigrettes  and 
effluves ;  or  else  one  had  to  deal  with  heated 
gases  difficult  to  handle,  which  were  confined  in 
receptacles  whose  walls  played  a  troublesome  part 
and  succeeded  in  veiling  the  simplicity  of  the  funda- 
mental facts.  Notwithstanding,  therefore,  the  efforts 
of  a  great  number  of  seekers,  no  general  idea  dis- 
engaged itself  out  of  a  mass  of  often  contradictory 
information. 

Many  physicists,  in  France  particularly,  discarded 
the  study  of  questions  which  seemed  so  confused,  and 
it  must  even  be  frankly  acknowledged  that  some 
among  them  had  a  really  unfounded  distrust  of 
certain  results  which  should  have  been  considered 
proved,  but  which  had  the  misfortune  to  be  in  con- 
tradiction with  the  theories  in  current  use.  All  the 
classic  ideas  relating  to  electrical  phenomena  led  to  the 
consideration  that  there  existed  a  perfect  symmetry 
between  the  two  electricities,  positive  and  negative. 
In  the  passing  of  electricity  through  gases  there  is 
manifested,  on  the  contrary,  an  evident  dissymmetry. 
The  anode  and  the  cathode  are  immediately  distin- 
guished in  a  tube  of  rarefied  gas  by  their  peculiar 
appearance;  and  the  conductivity  does  not  appear, 
under  certain  conditions,  to  be  the  same  for  the  two 
modes  of  electrification. 

It  is  not  devoid  of  interest  to  note  that  Erman,  a 
German scholar,once  very  celebrated  and  nowgenerally 


238    THE  NEW   PHYSICS  AND   ITS  EVOLUTION 

forgotten,  drew  attention  as  early  as  1815  to  the  uni- 
polar conductivity  of  a  flame.  His  contemporaries, 
as  may  be  gathered  from  the  perusal  of  the  treatises 
on  physics  of  that  period,  attached  great  importance 
to  this  discovery ;  but,  as  it  was  somewhat  inconveni- 
ent and  did  not  readily  fit  in  with  ordinary  studies,  it 
was  in  due  course  neglected,  then  considered  as  in- 
sufficiently established,  and  finally  wholly  forgotten. 

All  these  somewhat  obscure  facts,  and  some  others 
— such  as  the  different  action  of  ultra-violet  radia- 
tions on  positively  and  negatively  charged  bodies — 
are  now,  on  the  contrary,  about  to  be  co-ordinated, 
thanks  to  the  modern  ideas  on  the  mechanism  of 
conduction ;  while  these  ideas  will  also  allow  us  to 
interpret  the  most  striking  dissymmetry  of  all,  i.e. 
that  revealed  by  electrolysis  itself,  a  dissymmetry 
which  certainly  can  not  be  denied,  but  to  which 
sufficient  attention  has  not  been  given. 

It  is  to  a  German  physicist,  Giese,  that  we  owe 
the  first  notions  on  the  mechanism  of  the  conduc- 
tivity of  gases,  as  we  now  conceive  it.  In  two 
memoirs  published  in  1882  and  1889,  he  plainly 
arrives  at  the  conception  that  conduction  in  gases  is 
not  due  to  their  molecules,  but  to  certain  fragments 
of  them  or  to  ions.  Giese  was  a  forerunner,  but 
his  ideas  could  not  triumph  so  long  as  there  were 
no  means  of  observing  conduction  in  simple  circum- 
stances. But  this  means  has  now  been  supplied  in 
the  discovery  of  the  X  rays. 


CONDUCTIVITY  OF  GASES  AND   THE  IONS    239 

Suppose  we  pass  through  some  gas  at  ordinary 
pressure,  such  as  hydrogen,  a  pencil  of  X  rays. 
The  gas,  which  till  then  has  behaved  as  a  perfect 
insulator,1  suddenly  acquires  a  remarkable  con- 
ductivity. If  into  this  hydrogen  two  metallic 
electrodes  in  communication  with  the  two  poles  of 
a  battery  are  introduced,  a  current  is  set  up  in  very 
special  conditions  which  remind  us,  when  they  are 
checked  by  experiments,  of  the  mechanism  which 
allows  the  passage  of  electricity  in  electrolysis,  and 
which  is  so  well  represented  to  us  when  we  picture 
to  ourselves  this  passage  as  due  to  the  migration 
towards  the  electrodes,  under  the  action  of  the  field, 
of  the  two  sets  of  ions  produced  by  the  spontaneous 
division  of  the  molecule  within  the  solution. 

Let  us  therefore  recognise  with  J.  J.  Thomson 
and  the  many  physicists  who,  in  his  wake,  have 
taken  up  and  developed  the  idea  of  Giese,  that, 
under  the  influence  of  the  X  rays,  for  reasons  which 
will  have  to  be  determined  later,  certain  gaseous 
molecules  have  become  divided  into  two  portions, 
the  one  positively  and  the  other  negatively  electrified, 

1  At  least,  so  long  as  it  is  not  introduced  between  the  two 
coatings  of  a  condenser  having  a  difference  of  potential  suffi- 
cient to  overcome  what  M.  Bouty  calls  its  dielectric  cohesion. 
We  leave  on  one  side  this  phenomenon,  regarding  which 
M.  Bouty  has  arrived  at  extremely  important  results  by  a  very 
remarkable  series  of  experiments ;  but  this  question  rightly 
belongs  to  a  special  study  of  electrical  phenomena  which  is  not 
yet  written. 


240    THE   NEW   PHYSICS   AND  ITS  EVOLUTION 

which  we  will  call,  by  analogy  with  the  kindred 
phenomenon  in  electrolysis,  by  the  name  of  ions.  If 
the  gas  be  then  placed  in  an  electric  field,  produced, 
for  instance,  by  two  metallic  plates  connected  with 
the  two  poles  of  a  battery  respectively,  the  positive 
ions  will  travel  towards  the  plate  connected  with 
the  negative  pole,  and  the  negative  ions  in  the 
contrary  direction.  There  is  thus  produced  a 
current  due  to  the  transport  to  the  electrodes  of 
the  charges  which  existed  on  the  ions. 

If  the  gas  thus  ionised  be  left  to  itself,  in  the 
absence  of  any  electric  field,  the  ions,  yielding  to 
their  mutual  attraction,  must  finally  meet,  combine, 
and  reconstitute  a  neutral  molecule,  thus  returning 
to  their  initial  condition.  The  gas  in  a  short  while 
loses  the  conductivity  which  it  had  acquired;  or 
this  is,  at  least,  the  phenomenon  at  ordinary 
temperatures.  But  if  the  temperature  is  raised, 
the  relative  speeds  of  the  ions  at  the  moment  of 
impact  may  be  great  enough  to  render  it  impossible 
for  the  recombination  to  be  produced  in  its  entirety, 
and  part  of  the  conductivity  will  remain. 

Every  element  of  volume  rendered  a  conductor 
therefore  furnishes,  in  an  electric  field,  equal  quan- 
tities of  positive  and  negative  electricity.  If  we 
admit,  as  mentioned  above,  that  these  liberated 
quantities  are  borne  by  ions  each  bearing  an  equal 
charge,  the  number  of  these  ions  will  be  proportional 
to  the  quantity  of  electricity,  and  instead  of  speaking 


CONDUCTIVITY  OF  GASES  AND  THE   IONS    241 

of  a  quantity  of  electricity,  we  could  use  the  equiva- 
lent term  of  number  of  ions.  For  the  excitement 
produced  by  a  given  pencil  of  X  rays,  the  number 
of  ions  liberated  will  be  fixed.  Thus,  from  a  given 
volume  of  gas  there  can  only  be  extracted  an  equally 
determinate  quantity  of  electricity. 

The  conductivity  produced  is  not  governed  by 
Ohm's  law.  The  intensity  is  not  proportional  to 
the  electromotive  force,  and  it  increases  at  first  as 
the  electromotive  force  augments  ;  but  it  approaches 
asymptotically  to  a  maximum  value  which  corre- 
sponds to  the  number  of  ions  liberated,  and  can 
therefore  serve  as  a  measure  of  the  power  of  the 
excitement.  It  is  this  current  which  is  termed  the 
current  of  saturation. 

M.  Kighi  has  ably  demonstrated  that  ionised  gas 
does  not  obey  the  law  of  Ohm  by  an  experiment 
very  paradoxical  in  appearance.  He  found  that,  the 
greater  the  distance  of  the  two  electrode  plates  from 
each,  the  greater  may  be,  within  certain  limits,  the 
intensity  of  the  current.  The  fact  is  very  clearly 
interpreted  by  the  theory  of  ionisation,  since  the 
greater  the  length  of  the  gaseous  column  the  greater 
must  be  the  number  of  ions  liberated. 

One  of  the  most  striking  characteristics  of  ionised 
gases  is  that  of  discharging  electrified  conductors. 
This  phenomenon  is  not  produced  by  the  departure 
of  the  charge  that  these  conductors  may  possess, 
but  by  the  advent  of  opposite  charges  brought  to 


242    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

them  by  ions  which  obey  the  electrostatic  attraction 
and  abandon  their  own  electrification  when  they 
come  in  contact  with  these  conductors. 

This  mode  of  regarding  the  phenomena  is  ex- 
tremely convenient  and  eminently  suggestive.  It 
may,  no  doubt,  be  thought  that  the  image  of  the  ions 
is  not  identical  with  objective  reality,  but  we  are 
compelled  to  acknowledge  that  it  represents  with 
absolute  faithfulness  all  the  details  of  the  phenomena. 

Other  facts,  moreover,  will  give  to  this  hypothesis 
a  still  greater  value;  we  shall  even  be  able,  so  to 
speak,  to  grasp  these  ions  individually,  to  count  them, 
and  to  measure  their  charge. 

§  2.  THE  CONDENSATION  OF  WATER- VAPOUR 
BY  IONS 

If  the  pressure  of  a  vapour — that  of  water,  for 
instance — in  the  atmosphere  reaches  the  value  of 
the  maximum  pressure  corresponding  to  the  tem- 
perature of  the  experiment,  the  elementary  theory 
teaches  us  that  the  slightest  decrease  in  temperature 
will  induce  a  condensation;  that  small  drops  will 
form,  and  the  mist  will  turn  into  rain. 

In  reality,  matters  do  not  occur  in  so  simple  a 
manner.  A  more  or  less  considerable  delay  may 
take  place,  and  the  vapour  will  remain  supersatu- 
rated. We  easily  discover  that  this  phenomenon  is 
due  to  the  intervention  of  capillary  action.  On  a 
drop  of  liquid  a  surface-tension  takes  effect  which 


CONDUCTIVITY  OF  GASES  AND  THE  IONS    243 

gives  rise  to  a  pressure  which  becomes  greater  the 
smaller  the  diameter  of  the  drop. 

Pressure  facilitates  evaporation,  and  on  more 
closely  examining  this  reaction  we  arrive  at  the  con- 
clusion that  vapour  can  never  spontaneously  condense 
itself  when  liquid  drops  already  formed  are  not 
present,  unless  forces  of  another  nature  intervene  to 
diminish  the  effect  of  the  capillary  forces.  In  the 
most  frequent  cases,  these  forces  come  from  the 
dust  which  is  always  in  suspension  in  the  air,  or 
which  exists  in  any  recipient.  Grains  of  dust  act  by 
reason  of  their  hygrometrical  power,  and  form  germs 
round  which  drops  presently  form.  It  is  possible  to 
make  use,  as  did  M.  Coulier  as  early  as  1875,  of  this 
phenomenon  to  carry  off  the  germs  of  condensation, 
by  producing  by  expansion  in  a  bottle  containing  a 
little  water  a  preliminary  mist  which  purifies  the 
air.  In  subsequent  experiments  it  will  be  found 
almost  impossible  to  produce  further  condensation 
of  vapour. 

But  these  forces  may  also  be  of  electrical  origin. 
Von  Helmholtz  long  since  showed  that  electricity 
exercises  an  influence  on  the  condensation  of  the 
vapour  of  water,  and  Mr  C.  T.  E.  Wilson,  with  this 
view,  has  made  truly  quantitative  experiments.  It 
was  rapidly  discovered  after  the  apparition  of  the 
X  rays  that  gases  that  have  become  conductors,  that 
is,  ionised  gases,  also  facilitate  the  condensation  of 
supersaturated  water  vapour. 


244    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

We  are  thus  led  by  a  new  road  to  the  belief 
that  electrified  centres  exist  in  gases,  and  that  each 
centre  draws  to  itself  the  neighbouring  molecules  of 
water,  as  an  electrified  rod  of  resin  does  the  light 
bodies  around  it.  There  is  produced  in  this  manner 
round  each  ion  an  assemblage  of  molecules  of  water 
which  constitute  a  germ  capable  of  causing  the 
formation  of  a  drop  of  water  out  of  the  condensation 
of  excess  vapour  in  the  ambient  air.  As  might  be 
expected,  the  drops  are  electrified,  and  take  to  them- 
selves the  charge  of  the  centres  round  which  they 
are  formed ;  moreover,  as  many  drops  are  created 
as  there  are  ions.  Thereafter  we  have  only  to  count 
these  drops  to  ascertain  the  number  of  ions  which 
existed  in  the  gaseous  mass. 

To  effect  this  counting,  several  methods  have  been 
used,  differing  in  principle  but  leading  to  similar 
results.  It  is  possible,  as  Mr  C.  T.  E.  Wilson  and 
Professor  J.  J.  Thomson  have  done,  to  estimate,  on 
the  one  hand,  the  weight  of  the  mist  which  is  pro- 
duced in  determined  conditions,  and  on  the  other,  the 
average  weight  of  the  drops,  according  to  the  formula 
formerly  given  by  Sir  G.  Stokes,  by  deducting  their 
diameter  from  the  speed  with  which  this  mist  falls ; 
or  we  can,  with  Professor  Lemme,  determine  the 
average  radius  of  the  drops  by  an  optical  process, 
viz.  by  measuring  the  diameter  of  the  first  diffraction 
ring  produced  when  looking  through  the  mist  at  a 
point  of  light. 


CONDUCTIVITY  OF  GASES  AND  THE   IONS    245 

We  thus  get  to  a  very  high  number.  There  are, 
for  instance,  some  twenty  million  ions  per  centimetre 
cube  when  the  rays  have  produced  their  maximum 
effect,  but  high  as  this  figure  is,  it  is  still  very  small 
compared  with  the  total  number  of  molecules.  All 
conclusions  drawn  from  kinetic  theory  lead  us  to 
think  that  in  the  same  space  there  must  exist,  by 
the  side  of  a  molecule  divided  into  two  ions,  a 
thousand  millions  remaining  in  a  neutral  state  and 
intact. 

Mr  C.  T.  E.  Wilson  has  remarked  that  the  positive 
and  negative  ions  do  not  produce  condensation  with 
the  same  facility.  The  ions  of  a  contrary  sign  may 
be  almost  completely  separated  by  placing  the 
ionised  gas  in  a  suitably  disposed  field.  In  the 
neighbourhood  of  a  negative  disk  there  remain 
hardly  any  but  positive  ions,  and  against  a  positive 
disk  none  but  negative ;  and  in  effecting  a  separation 
of  this  kind,  it  will  be  noticed  that  condensation  by 
negative  ions  is  easier  than  by  the  positive. 

It  is,  consequently,  possible  to  cause  condensation 
on  negative  centres  only,  and  to  study  separately  the 
phenomena  produced  by  the  two  kinds  of  ions.  It 
can  thus  be  verified  that  they  really  bear  charges 
equal  in  absolute  value,  and  these  charges  can  even 
be  estimated,  since  we  already  know  the  number  of 
drops.  This  estimate  can  be  made,  for  example,  by 
comparing  the  speed  of  the  fall  of  a  mist  in  fields 
of  different  values,  or,  as  did  J.  J.  Thomson,  by 


246    THE  NEW  PHYSICS   AND  ITS  EVOLUTION 

measuring  the  total  quantity  of  electricity  liberated 
throughout  the  gas. 

At  the  degree  of  approximation  which  such 
experiments  imply,  we  find  that  the  charge  of  a 
drop,  and  consequently  the  charge  borne  by  an  ion,  is 
sensibly  34  X  10~10  electrostatic  or  11  x  10~20  electro- 
magnetic units.  This  charge  is  very  near  that  which 
the  study  of  the  phenomena  of  ordinary  electrolysis 
leads  us  to  attribute  to  a  univalent  atom  produced 
by  electrolytic  dissociation. 

Such  a  coincidence  is  evidently  very  striking  ; 
but  it  will  not  be  the  only  one,  for  whatever 
phenomenon  be  studied  it  will  always  appear  that 
the  smallest  charge  we  can  conceive  as  isolated  is 
that  mentioned.  We  are,  in  fact,  in  presence  of  a 
natural  unit,  or,  if  you  will,  of  an  atom  of  electricity. 

We  must,  however,  guard  against  the  belief  that 
the  gaseous  ion  is  identical  with  the  electrolytic  ion. 
Sensible  differences  between  those  are  immediately 
apparent,  and  still  greater  ones  will  be  discovered  on 
closer  examination. 

As  M.  Perrin  has  shown,  the  ionisation  produced 
by  the  X  rays  in  no  way  depends  on  the  chemical 
composition  of  the  gas ;  and  whether  we  take  a 
volume  of  gaseous  hydrochloric  acid  or  a  mixture  of 
hydrogen  and  chlorine  in  the  same  condition,  all  the 
results  will  be  identical :  and  chemical  affinities  play 
no  part  here. 

We  can  also  obtain  other  information  regarding 


CONDUCTIVITY  OF  GASES  AND  THE   IONS     247 

ions :  we  can  ascertain,  for  instance,  their  velocities, 
and  also  get  an  idea  of  their  order  of  magnitude. 

By  treating  the  speeds  possessed  by  the  liberated 
charges  as  components  of  the  known  speed  of  a 
gaseous  current,  Mr  Zeleny  measures  the  mobilities, 
that  is  to  say,  the  speeds  acquired  by  the  positive  and 
negative  charges  in  a  field  equal  to  the  electrostatic 
unit.  He  has  thus  found  that  these  mobilities 
are  different,  and  that  they  vary,  for  example, 
between  400  and  200  centimetres  per  second  for  the 
two  charges  in  dry  gases,  the  positive  being  less 
mobile  than  the  negative  ions,  which  suggests  the 
idea  that  they  are  of  greater  mass.1 

M.  Langevin,  who  has  made  himself  the  eloquent 
apostle  of  the  new  doctrines  in  France,  and  has 
done  much  to  make  them  understood  and  admitted, 
has  personally  undertaken  experiments  analogous  to 
those  of  M.  Zeleny,  but  much  more  complete.  He 
has  studied  in  a  very  ingenious  manner,  not  only 
the  mobilities,  but  also  the  law  of  recombination 
which  regulates  the  spontaneous  return  of  the  gas 
to  its  normal  state.  He  has  determined  experiment- 
ally the  relation  of  the  number  of  recombinations 
to  the  number  of  collisions  between  two  ions  of 
contrary  sign,  by  studying  the  variation  produced 
by  a  change  in  the  value  of  the  field,  in  the 

1  A  full  account  of  these  experiments,  which  were  executed 
at  the  Cavendish  Laboratory,  is  to  be  found  in  Philosophical 
Transactions,  A.,  vol.  cxcv.  (1901),  pp.  193  et  seq. — ED. 


248    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

quantity  of  electricity  which  can  be  collected  in 
the  gas  separating  two  parallel  metallic  plates, 
after  the  passage  through  it  for  a  very  short  time 
of  the  Kontgen  rays  emitted  during  one  discharge 
of  a  Crookes  tube.  If  the  image  of  the  ions  is 
indeed  conformable  to  reality,  this  relation  must 
evidently  always  be  smaller  than  unity,  and  must 
tend  towards  this  value  when  the  mobility  of  the 
ions  diminishes,  that  is  to  say,  when  the  pressure 
of  the  gas  increases.  The  results  obtained  are  in 
perfect  accord  with  this  anticipation. 

On  the  other  hand,  M.  Langevin  has  succeeded,  by 
following  the  displacement  of  the  ions  between  the 
parallel  plates  after  the  ionisation  produced  by  the 
radiation,  in  determining  the  absolute  values  of 
the  mobilities  with  great  precision,  and  has  thus 
clearly  placed  in  evidence  the  irregularity  of  the 
mobilities  of  the  positive  and  negative  ions  respec- 
tively. Their  mass  can  be  calculated  when  we 
know,  through  experiments  of  this  kind,  the  speed  of 
the  ions  in  a  given  field,  and  on  the  other  hand — 
as  we  can  now  estimate  their  electric  charge — the 
force  which  moves  them.  They  evidently  progress 
more  slowly  the  larger  they  are ;  and  in  the  viscous 
medium  constituted  by  the  gas,  the  displacement  is 
effected  at  a  speed  sensibly  proportional  to  the 
motive  power. 

At  the  ordinary  temperature  these  masses  are 
relatively  considerable,  and  are  greater  for  the 


CONDUCTIVITY  OF  GASES  AND  THE  IONS    249 

positive  than  for  the  negative  ions,  that  is  to  say, 
they  are  about  the  order  of  some  ten  molecules.  The 
ions,  therefore,  seem  to  be  formed  by  an  agglomeration 
of  neutral  molecules  maintained  round  an  electrified 
centre  by  electrostatic  attraction.  If  the  tempera- 
ture rises,  the  thermal  agitation  will  become  great 
enough  to  prevent  the  molecules  from  remaining 
linked  to  the  centre.  By  measurements  effected  on 
the  gases  of  flames,  we  arrive  at  very  different  values 
of  the  masses  from  those  found  for  ordinary  ions,  and 
above  all,  very  different  ones  for  ions  of  contrary 
sign.  The  negative  ions  have  much  more  consider- 
able velocities  than  the  positive  ones.  The  latter 
also  seem  to  be  of  the  same  size  as  atoms;  and 
the  first-named  must,  consequently,  be  considered  as 
very  much  smaller,  and  probably  about  a  thousand 
times  less. 

Thus,  for  the  first  time  in  science,  the  idea  appears 
that  the  atom  is  not  the  smallest  fraction  of  matter 
to  be  considered.  Fragments  a  thousand  times 
smaller  may  exist  which  possess,  however,  a  negative 
charge.  These  are  the  electrons,  which  other  con- 
siderations will  again  bring  to  our  notice. 

§  3.    HOW   IONS  ARE  PRODUCED 

It  is  very  seldom  that  a  gaseous  mass  does  not 
contain  a  few  ions.  They  may  have  been  formed 
from  many  causes,  for  although  to  give  precision  to 
our  studies,  and  to  deal  with  a  well  ascertained  case, 


250    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

I  mentioned  only  ionisation  by  the  X  rays  in  the 
first  instance,  I  ought  not  to  give  the  impression 
that  the  phenomenon  is  confined  to  these  rays.  It 
is,  on  the  contrary,  very  general,  and  ionisation  is 
just  as  well  produced  by  the  cathode  rays,  by  the 
radiations  emitted  by  radio-active  bodies,  by  the 
ultra-violet  rays,  by  heating  to  a  high  temperature, 
by  certain  chemical  actions,  and  finally  by  the  impact 
of  the  ions  already  existing  in  neutral  molecules. 

Of  late  years  these  new  questions  have  been  the 
object  of  a  multitude  of  researches,  and  if  it  has  not 
always  been  possible  to  avoid  some  confusion,  yet 
certain  general  conclusions  may  be  drawn.  The  ion- 
isation by  flames,  in  particular,  is  fairly  well  known. 
For  it  to  be  produced  spontaneously,  it  would  appear 
that  there  must  exist  simultaneously  a  rather  high 
temperature  and  a  chemical  action  in  the  gas.  Ac- 
cording to  M.  Moreau,  the  ionisation  is  very  marked 
when  the  flame  contains  the  vapour  of  the  salt  of 
an  alkali  or  of  an  alkaline  earth,  but  much  less  so 
when  it  contains  that  of  other  salts.  Arrhenius, 
Mr  C.  T.  K.  Wilson,  and  M.  Moreau,  have  studied  all 
the  circumstances  of  the  phenomenon ;  and  it  seems 
indeed  that  there  is  a  somewhat  close  analogy 
between  what  first  occurs  in  the  saline  vapours  and 
that  which  is  noted  in  liquid  electrolytes.  There 
should  be  produced,  as  soon  as  a  certain  temperature 
is  reached,  a  dissociation  of  the  saline  molecule  ;  and, 
as  M.  Moreau  has  shown  in  a  series  of  very  well  con- 


CONDUCTIVITY  OF  GASES  AND  THE   IONS    251 

ducted  researches,  the  ions  formed  at  about  100°  C. 
seem  constituted  by  an  electrified  centre  of  the  size 
of  a  gas  molecule,  surrounded  by  some  ten  layers  of 
other  molecules.  We  are  thus  dealing  with  rather 
large  ions,  but  according  to  Mr  Wilson,  this  conden- 
sation phenomenon  does  not  affect  the  number  of  ions 
produced  by  dissociation.  In  proportion  as  the 
temperature  rises,  the  molecules  condensed  round  the 
nucleus  disappear,  and,  as  in  all  other  circumstances, 
the  negative  ion  tends  to  become  an  electron,  while 
the  positive  ion  continues  the  size  of  an  atom. 

In  other  cases,  ions  are  found  still  larger  than 
those  of  saline  vapours,  as,  for  example,  those  pro- 
duced by  phosphorus.  It  has  long  been  known  that 
air  in  the  neighbourhood  of  phosphorus  becomes  a 
conductor,  and  the  fact,  pointed  out  as  far  back  as 
1885  by  Matteucci,  has  been  well  studied  by  various 
experimenters,  by  MM.  Elster  and  Geitel  in  1890, 
for  instance.  On  the  other  hand,  in  1893  Mr  Barus 
established  that  the  approach  of  a  stick  of  phos- 
phorus brings  about  the  condensation  of  water 
vapour,  and  we  really  have  before  us,  therefore,  in 
this  instance,  an  ionisation.  M.  Bloch  has  succeeded 
in  disentangling  the  phenomena,  which  are  here  very 
complex,  and  in  showing  that  the  ions  produced  are 
of  considerable  dimensions;  for  their  speed  in  the 
same  conditions  is  on  the  average  a  thousand  times 
less  than  that  of  ions  due  to  the  X  rays.  M.  Bloch 
has  established  also  that  the  conductivity  of  recently- 


252     THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

prepared  gases,  already  studied  by  several  authors, 
was  analogous  to  that  which  is  produced  by 
phosphorus,  and  that  it  is  intimately  connected 
with  the  presence  of  the  very  tenuous  solid  or  liquid 
dust  which  these  gases  carry  with  them,  while  the 
ions  are  of  the  same  order  of  magnitude.  These 
large  ions  exist,  moreover,  in  small  quantities  in 
the  atmosphere ;  and  M.  Langevin  lately  succeeded 
in  revealing  their  presence. 

It  may  happen,  and  this  not  without  singularly 
complicating  matters,  that  the  ions  which  were  in 
the  midst  of  material  molecules  produce,  as  the 
result  of  collisions,  new  divisions  in  these  last. 
Other  ions  are  thus  born,  and  this  production 
is  in  part  compensated  for  by  recombinations 
between  ions  of  opposite  signs.  The  impacts  will 
be  more  active  in  the  event  of  the  gas  being  placed 
in  a  field  of  force  and  of  the  pressure  being  slight, 
the  speed  attained  being  then  greater  and  allowing 
the  active  force  to  reach  a  high  value.  The  energy 
necessary  for  the  production  of  an  ion  is,  in  fact, 
according  to  Professor  Eutherford  and  Professor 
Stark,  something  considerable,  and  it  much  exceeds 
the  analogous  force  in  electrolytic  decomposition. 

It  is  therefore  in  tubes  of  rarefied  gas  that  this 
ionisation  by  impact  will  be  particularly  felt.  This 
gives  us  the  reason  for  the  aspect  presented  by 
Geissler  tubes.  Generally,  in  the  case  of  discharges, 
new  ions  produced  by  the  molecules  struck  come  to 


CONDUCTIVITY  OF  GASES  AND  THE  IONS    253 

add  themselves  to  the  electrons  produced,  as  will 
be  seen,  by  the  cathode.  A  full  discussion  has  led 
to  the  interpretation  of  all  the  known  facts,  and 
to  our  understanding,  for  instance,  why  there  exist 
bright  or  dark  spaces  in  certain  regions  of  the  tube. 
M.  Pell  at,  in  particular,  has  given  some  very  fine 
examples  of  this  concordance  between  the  theory 
and  the  facts  he  has  skilfully  observed. 

In  all  the  circumstances,  then,  in  which  ions 
appear,  their  formation  has  doubtless  been  provoked 
by  a  mechanism  analogous  to  that  of  the  shock. 
The  X  rays,  if  they  are  attributable  to  sudden 
variations  in  the  ether — that  is  to  say,  a  variation 
of  the  two  vectors  of  Hertz — themselves  produce 
within  the  atom  a  kind  of  electric  impulse  which 
breaks  it  into  two  electrified  fragments;  i.e.  the 
positive  centre,  the  size  of  the  molecule  itself,  and 
the  negative  centre,  constituted  by  an  electron  a 
thousand  times  smaller.  Eound  these  two  centres, 
at  the  ordinary  temperature,  are  agglomerated  by 
attraction  other  molecules,  and  in  this  manner  the 
ions  whose  properties  have  just  been  studied  are 
formed. 

§  4.  ELECTRONS  IN  METALS 

The  success  of  the  ionic  hypothesis  as  an  inter- 
pretation of  the  conductivity  of  electrolytes  and 
gases  has  suggested  the  desire  to  try  if  a  similar 
hypothesis  can  represent  the  ordinary  conductivity 


254    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

of  metals.  We  are  thus  led  to  conceptions  which 
at  first  sight  seem  audacious  because  they  are 
contrary  to  our  habits  of  mind.  They  must  not, 
however,  be  rejected  on  that  account.  Electrolytic 
dissociation  at  first  certainly  appeared  at  least  as 
strange;  yet  it  has  ended  by  forcing  itself  upon 
us,  and  we  could,  at  the  present  day,  hardly  dis- 
pense with  the  image  it  presents  to  us. 

The  idea  that  the  conductivity  of  metals  is  not 
essentially  different  from  that  of  electrolytic  liquids 
or  gases,  in  the  sense  that  the  passage  of  the  current 
is  connected  with  the  transport  of  small  electrified 
particles,  is  already  of  old  date.  It  was  enunciated 
by  W.  Weber,  and  afterwards  developed  by  Giese, 
but  has  only  obtained  its  true  scope  through  the 
effect  of  recent  discoveries  It  was  the  researches 
of  Eiecke,  later,  of  Drude,  and,  above  all,  those  of 
J.  J.  Thomson,  which  have  allowed  it  to  assume  an 
acceptable  form.  All  these  attempts  are  connected 
however  with  the  general  theory  of  Lorentz,  which 
we  will  examine  later. 

It  will  be  admitted  that  metallic  atoms  can,  like 
the  saline  molecule  in  a  solution,  partially  disso- 
ciate themselves.  Electrons,  very  much  smaller  than 
atoms,  can  move  through  the  structure,  considerable 
to  them,  which  is  constituted  by  the  atom  from 
which  they  have  just  been  detached.  They  may  be 
compared  to  the  molecules  of  a  gas  which  is  enclosed 
in  a  porous  body.  In  ordinary  conditions,  notwith- 


CONDUCTIVITY  OF  GASES   AND   THE   IONS    255 

standing  the  great  speed  with  which  they  are 
animated,  they  are  unable  to  travel  long  distances, 
because  they  quickly  find  their  road  barred  by  a 
material  atom.  They  have  to  undergo  innumerable 
impacts,  which  throw  them  first  in  one  direction 
and  then  in  another.  The  passage  of  a  current  is 
a  sort  of  flow  of  these  electrons  in  a  determined 
direction.  This  electric  flow  brings,  however,  no 
modification  to  the  material  medium  traversed, 
since  every  electron  which  disappears  at  any  point 
is  replaced  by  another  which  appears  at  once,  and 
in  all  metals  the  electrons  are  identical. 

This  hypothesis  leads  us  to  anticipate  certain  facts 
which  experience  confirms.  Thus  J.  J.  Thomson 
shows  that  if,  in  certain  conditions,  a  conductor  is 
placed  in  a  magnetic  field,  the  ions  have  to  describe 
an  epicycloid,  and  their  journey  is  thus  lengthened, 
while  the  electric  resistance  must  increase.  If  the 
field  is  in  the  direction  of  the  displacement,  they 
describe  helices  round  the  lines  of  force  and  the 
resistance  is  again  augmented,  but  in  different 
proportions.  Various  experimenters  have  noted 
phenomena  of  this  kind  in  different  substances. 

For  a  long  time  it  has  been  noticed  that  a  relation 
exists  between  the  calorific  and  the  electric  con- 
ductivity ;  the  relation  of  these  two  conductivities 
is  sensibly  the  same  for  all  metals.  The  modern 
theory  tends  to  show  simply  that  it  must  indeed  be 
so.  Calorific  conductivity  is  due,  in  fact,  to  an 


256    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

exchange  of  electrons  between  the  hot  and  the  cold 
regions,  the  heated  electrons  having  the  greater 
velocity,  and  consequently  the  more  considerable 
energy.  The  calorific  exchanges  then  obey  laws 
similar  to  those  which  govern  electric  exchanges  ; 
and  calculation  even  leads  to  the  exact  values  which 
the  measurements  have  given.1 

In  the  same  way  Professor  Hesehus  has  explained 
how  contact  electrification  is  produced,  by  the  ten- 
dency of  bodies  to  equalise  their  superficial  properties 
by  means  of  a  transport  of  electrons,  and  Mr  Jeans 
has  shown  that  we  should  discover  the  existence  of 
the  well-known  laws  of  distribution  over  conducting 
bodies  in  electrostatic  equilibrium.  A  metal  can, 
in  fact,  be  electrified,  that  is  to  say,  may  possess  an 
excess  of  positive  or  negative  electrons  which  cannot 
easily  leave  it  in  ordinary  conditions.  To  cause 
them  to  do  so  would  need  an  appreciable  amount  of 
work,  on  account  of  the  enormous  difference  of  the 
specific  inductive  capacities  of  the  metal  and  of 
the  insulating  medium  in  which  it  is  plunged. 

Electrons,  however,  which,  on  arriving  at  the  sur- 
face of  the  metal,  possessed  a  kinetic  energy 
superior  to  this  work,  might  be  shot  forth  and  would 
be  disengaged  as  a  vapour  escapes  from  a  liquid. 

1  The  whole  of  this  argument  is  brilliantly  set  forth  by 
Professor  Lorentz  in  a  lecture  delivered  to  the  Electrotechniker- 
verein  at  Berlin  in  December  1904,  and  reprinted,  with 
additions,  in  the  Archives  NJerlandaises  of  1906.— ED. 


CONDUCTIVITY  OF  GASES  AND  THE   IONS    257 

Now,  the  number  of  these  rapid  electrons,  at  first 
very  slight,  increases,  according  to  the  kinetic 
theory,  when  the  temperature  rises,  and  therefore  we 
must  reckon  that  a  wire,  on  being  heated,  gives  out 
electrons,  that  is  to  say,  loses  negative  electricity 
and  sends  into  the  surrounding  media  electrified 
centres  -capable  of  producing  the  phenomena  of 
ionisation.  Edison,  in  1884,  showed  that  from  the 
filament  of  an  incandescent  lamp  there  escaped 
negative  electric  charges.  Since  then,  Richardson  and 
J.  J.  Thomson  have  examined  analogous  phenomena. 
This  emission  is  a  very  general  phenomenon 
which,  no  doubt,  plays  a  considerable  part  in  cosmic 
physics.  Professor  Arrhenius  explains,  for  instance, 
the  polar  auroras  by  the  action  of  similar  corpuscules 
emitted  by  the  sun. 

In  other  phenomena  we  seem  indeed  to  be  con- 
fronted by  an  emission,  not  of  negative  electrons, 
but  of  positive  ions.  Thus,  when  a  wire  is  heated, 
not  in  vacuo,  but  in  a  gas,  this  wire  begins  to  electrify 
neighbouring  bodies  positively.  J.  J.  Thomson  has 
measured  the  mass  of  these  positive  ions  and  finds 
it  considerable,  i.e.  about  150  times  that  of  an  atom 
of  hydrogen.  Some  are  even  larger,  and  constitute 
almost  a  real  grain  of  dust.  We  here  doubtless 
meet  with  the  phenomena  of  disaggregation  under- 
gone by  metals  at  a  red  heat. 


T7 


CHAPTER  IX 

CATHODE  RAYS  AND   RADIOACTIVE 
BODIES 

§  1.  THE  CATHODE  BAYS 

A  WIRE  traversed  by  an  electric  current  is,  as  has 
just  been  explained,  the  seat  of  a  movement  of 
electrons.  If  we  cut  this  wire,  a  flood  of  electrons, 
like  a  current  of  water  which,  at  the  point  where  a 
pipe  bursts,  flows  out  in  abundance,  will  appear  to 
spring  out  between  the  two  ends  of  the  break. 

If  the  energy  of  the  electrons  is  sufficient,  these 
electrons  will  in  fact  rush  forth  and  be  propagated 
in  the  air  or  in  the  insulating  medium  interposed  ; 
but  the  phenomena  of  the  discharge  will  in  general 
be  very  complex.  We  shall  here  only  examine  a 
particularly  simple  case,  viz.,  that  of  the  cathode 
rays;  and  without  entering  into  details,  we  shall 
only  note  the  results  relating  to  these  rays  which 
furnish  valuable  arguments  in  favour  of  the  electronic 
hypothesis  and  supply  solid  materials  for  the  con- 
struction of  new  theories  of  electricity  and  matter. 

258 


CATHODE  EAYS  AND  RADIOACTIVE   BODIES    259 

For  a  long  time  it  was  noticed  that  the  phenomena 
in  a  Geissler  tube  changed  their  aspect  considerably, 
when  the  gas  pressure  became  very  weak,  without, 
however,  a  complete  vacuum  being  formed.  From 
the  cathode  there  is  shot  forth  normally  and  in  a 
straight  line  a  flood  within  the  tube,  dark  but 
capable  of  impressing  a  photographic  plate,  of 
developing  the  fluorescence  of  various  substances 
(particularly  the  glass  walls  of  the  tube),  and  of 
producing  calorific  and  mechanical  effects.  These 
are  the  cathode  rays,  so  named  in  1883  by  E. 
Wiedemann,  and  their  name,  which  was  unknown 
to  a  great  number  of  physicists-  till  barely  twelve 
years  ago,  has  become  popular  at  the  present 
day. 

About  1869,  Hittorf  made  an  already  very  com- 
plete study  of  them  and  put  in  evidence  their 
principal  properties ;  but  it  was  the  researches  of 
Sir  W.  Crookes  in  especial  which  drew  attention  to 
them.  The  celebrated  physicist  foresaw  that  the 
phenomena  which  were  thus  produced  in  rarefied 
gases  were,  in  spite  of  their  very  great  complica- 
tion, more  simple  than  those  presented  by  matter 
under  the  conditions  in  which  it  is  generally 
met  with. 

He  devised  a  celebrated  theory  no  longer  ad- 
missible in  its  entirety,  because  it  is  not  in  complete 
accord  with  the  facts,  which  was,  however,  very 
interesting,  and  contained,  in  germ,  certain  of  our 


26o    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

present  ideas.  In  the  opinion  of  Crookes,  in  a  tube 
in  which  the  gas  has  been  rarefied  we  are  in  presence 
of  a  special  state  of  matter.  The  number  of  the  gas 
molecules  has  become  small  enough  for  their  inde- 
pendence to  be  almost  absolute,  and  they  are  able  in 
this  so-called  radiant  state  to  traverse  long  spaces 
without  departing  from  a  straight  line.  The  cathode 
rays  are  due  to  a  kind  of  molecular  bombardment  of 
the  walls  of  the  tubes,  and  of  the  screens  which  can 
be  introduced  into  them;  and  it  is  the  molecules, 
electrified  by  their  contact  with  the  cathode  and  then 
forcibly  repelled  by  electrostatic  action,  which  pro- 
duce, by  their  movement  and  their  vis  viva,  all  the 
phenomena  observed.  Moreover,  these  electrified 
molecules  animated  with  extremely  rapid  velocities 
correspond,  according  to  the  theory  verified  in  the 
celebrated  experiment  of  Rowland  on  convection 
currents,  to  a  true  electric  current,  and  can  be 
deviated  by  a  magnet. 

Notwithstanding  the  success  of  Crookes'  experi- 
ments, many  physicists — the  Germans  especially — 
did  not  abandon  an  hypothesis  entirely  different 
from  that  of  radiant  matter.  They  continued  to 
regard  the  cathode  radiation  as  due  to  particular 
radiations  of  a  nature  still  little  known  but  pro- 
duced in  the  luminous  ether.  This  interpretation 
seemed,  indeed,  in  1894,  destined  to  triumph  de- 
finitely through  the  remarkable  discovery  of  Lenard, 
a  discovery  which,  in  its  turn,  was  to  provoke  so  many 


CATHODE   RAYS  AND   RADIOACTIVE   BODIES    261 

others  and  to  bring  about  consequences  of  which  the 
importance  seems  every  day  more  considerable. 

Professor  Lenard's  fundamental  idea  was  to  study 
the  cathode  rays  under  conditions  different  from  those 
in  which  they  are  produced.  These  rays  are  born 
in  a  very  rarefied  space,  under  conditions  perfectly 
determined  by  Sir  W.  Crookes ;  but  it  was  a  question 
whether,  when  once  produced,  they  would  be  capable 
of  propagating  themselves  in  other  media,  such  as  a 
gas  at  ordinary  pressure, or  even  in  an  absolute  vacuum. 
Experiment  alone  could  answer  this  question,  but 
there  were  difficulties  in  the  way  of  this  which 
seemed  almost  insurmountable.  The  rays  are  stopped 
by  glass  even  of  slight  thickness,  and  how  then 
could  the  almost  vacuous  space  in  which  they  have 
to  come  into  existence  be  separated  from  the  space, 
absolutely  vacuous  or  filled  with  gas,  into  which  it 
was  desired  to  bring  them  ? 

The  artifice  used  was  suggested  to  Professor  Lenard 
by  an  experiment  of  Hertz.  The  great  physicist 
had,  in  fact,  shortly  before  his  premature  death, 
taken  up  this  important  question  of  the  cathode 
rays,  and  his  genius  left  there,  as  elsewhere,  its 
powerful  impress.  He  had  shown  that  metallic 
plates  of  very  slight  thickness  were  transparent  to 
the  cathode  rays ;  and  Professor  Lenard  succeeded  in 
obtaining  plates  impermeable  to  air,  but  which  yet 
allowed  the  pencil  of  cathode  rays  to  pass  through 
them. 


262    THE  NEW  PHYSICS  AND   ITS   EVOLUTION 

Now  if  we  take  a  Crookes  tube  with  the  extremity 
hermetically  closed  by  a  metallic  plate  with  a  slit 
across  the  diameter  of  1  mm.  in  width,  and  stop 
this  slit  with  a  sheet  of  very  thin  aluminium,  it  will 
be  immediately  noticed  that  the  rays  pass  through 
the  aluminium  and  pass  outside  the  tube.  They 
are  propagated  in  air  at  atmospheric  pressure,  and 
they  can  also  penetrate  into  an  absolute  vacuum. 
They  therefore  can  no  longer  be  attributed  to  radiant 
matter,  and  we  are  led  to  think  that  the  energy 
brought  into  play  in  this  phenomenon  must  have 
its  seat  in  the  light-bearing  ether  itself. 

But  it  is  a  very  strange  light  which  is  thus 
subject  to  magnetic  action,  which  does  not  obey 
the  principle  of  equal  angles,  and  for  which  the 
most  various  gases  are  already  disturbed  media. 
According  to  Crookes  it  possesses  also  the  singular 
property  of  carrying  with  it  electric  charges. 

This  convection  of  negative  electricity  by  the 
cathode  rays  seems  quite  inexplicable  on  the  hypo- 
thesis that  the  rays  are  ethereal  radiations.  Nothing 
then  remained  in  order  to  maintain  this  hypothesis, 
except  to  deny  the  convection,  which,  besides,  was 
only  established  by  indirect  experiments.  That  the 
reality  of  this  transport  has  been  placed  beyond 
dispute  by  means  of  an  extremely  elegant  experi- 
ment which  is  all  the  more  convincing  that  it  is  so 
very  simple,  is  due  to  M.  Perrin.  In  the  interior 
of  a  Crookes  tube  he  collected  a  pencil  of  cathode 


CATHODE  BAYS  AND  RADIOACTIVE   BODIES    263 

rays  in  a  metal  cylinder.  According  to  the  ele- 
mentary principles  of  electricity  the  cylinder  must 
become  charged  with  the  whole  charge,  if  there  be 
one,  brought  to  it  by  the  rays,  and  naturally  various 
precautions  had  to  be  taken.  But  the  result  was 
very  precise,  and  doubt  could  no  longer  exist — the 
rays  were  electrified. 

It  might  have  been,  and  indeed  was,  maintained, 
some  time  after  this  experiment  was  published,  that 
while  the  phenomena  were  complex  inside  the  tube, 
outside,  things  might  perhaps  occur  differently. 
Lenard  himself,  however,  with  that  absence  of  even 
involuntary  prejudice  common  to  all  great  minds, 
undertook  to  demonstrate  that  the  opinion  he  at 
first  held  could  no  longer  be  accepted,  and  succeeded 
in  repeating  the  experiment  of  M.  Perrin  on 
cathode  rays  in  the  air  and  even  in  -vacuo. 

On  the  wrecks  of  the  two  contradictory  hypotheses 
thus  destroyed,  and  out  of  the  materials  from 
which  they  had  been  built,  a  theory  has  been  con- 
structed which  co-ordinates  all  the  known  facts. 
This  theory  is  furthermore  closely  allied  to  the 
theory  of  ionisation,  and,  like  this  latter,  is  based 
on  the  concept  of  the  electron.  Cathode  rays  are 
electrons  in  rapid  motion. 

The  phenomena  produced  both  inside  and  outside 
a  Crookes  tube  are,  however,  generally  complex.  In 
Lenard's  first  experiments,  and  in  many  others 
effected  later  when  this  region  of  physics  was  still 


264    THE  NEW  PHYSICS  AND  ITS   EVOLUTION 

very  little  known,  a  few  confusions  may  be  noticed 
even  at  the  present  day. 

At  the  spot  where  the  cathode  rays  strike  the 
walls  of  the  tube  the  essentially  different  X  rays 
appear.  These  differ  from  the  cathode  radiations  by 
being  neither  electrified  nor  deviated  by  a  magnet. 
In  their  turn  these  X  rays  may  give  birth  to  the 
secondary  rays  of  M.  Sagnac ;  and  often  we  find 
ourselves  in  presence  of  effects  from  these  last- 
named  radiations  and  not  from  the  true  cathode 
rays. 

The  electrons,  when  they  are  propagated  in  a 
gas,  can  ionise  the  molecules  of  this  gas  and  unite 
with  the  neutral  atoms  to  form  negative  ions,  while 
positive  ions  also  appear.  There  are  likewise 
produced,  at  the  expense  of  the  gas  still  subsisting 
after  rarefication  within  the  tube,  positive  ions 
which,  attracted  by  the  cathode  and  reaching  it,  are 
not  all  neutralised  by  the  negative  electrons,  and  can, 
if  the  cathode  be  perforated,  pass  through  it,  and  if 
not,  pass  round  it.  We  have  then  what  are  called 
the  canal  rays  of  Goldstein,  which  are  deviated  by 
an  electric  or  magnetic  field  in  a  contrary  direc- 
tion to  the  cathode  rays ;  but,  being  larger,  give 
weak  deviations  or  may  even  remain  undeviated 
through  losing  their  charge  when  passing  through 
the  cathode. 

It  may  also  be  the  parts  of  the  walls  at  a  distance 
from  the  cathode  which  send  a  positive  rush  to 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    265 

the  latter,  by  a  similar  mechanism.  It  may  be, 
again,  that  in  certain  regions  of  the  tube  cathode 
rays  are  met  with  diffused  by  some  solid  object, 
without  having  thereby  changed  their  nature. 
All  these  complexities  have  been  cleared  up  by 
M.  Villard,  who  has  published,  on  these  questions, 
some  remarkably  ingenious  and  particularly  careful 
experiments. 

M.  Villard  has  also  studied  the  phenomena  of  the 
coiling  of  the  rays  in  a  field,  as  already  pointed  out 
by  Hittorf  and  Pliicker.  When  a  magnetic  field 
acts  on  the  cathode  particle,  the  latter  follows  a 
trajectory,  generally  helicoidal,  which  is  anticipated 
by  the  theory.  We  here  have  to  do  with  a  question 
of  ballistics,  and  experiments  duly  confirm  the 
anticipations  of  the  calculation.  Nevertheless,  rather 
singular  phenomena  appear  in  the  case  of  certain 
values  of  the  field,  and  these  phenomena,  dimly  seen 
by  Pliicker  and  Birkeland,  have  been  the  object  of 
experiments  by  M.  Villard.  The  two  faces  of  the 
cathode  seem  to  emit  rays  which  are  deviated  in 
a  direction  perpendicular  to  the  lines  of  force  by 
an  electric  field,  and  do  not  seem  to  be  electrified. 
M.  Villard  calls  them  magneto-cathode  rays,  and 
according  to  M.  Fortin  these  rays  may  be  ordinary 
cathode  rays,  but  of  very  slight  velocity. 

In  certain  cases  the  cathode  itself  may  be  super- 
ficially disaggregated,  and  extremely  tenuous  particles 
detach  themselves,  which,  being  carried  off  at  right 


266    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

angles  to  its  surface,  may  deposit  themselves  like  a 
very  thin  film  on  objects  placed  in  their  path.  Various 
physicists,  among  them  M.  Houllevigue,  have  studied 
this  phenomenon,  and  in  the  case  of  pressures  between 
2\y  and  TJ0-  of  a  millimetre,  the  last-named  scholar 
has  obtained  mirrors  of  most  metals,  a  phenomenon 
he  designates  by  the  name  of  ionoplasty. 

But  in  spite  of  all  these  accessory  phenomena, 
which  even  sometimes  conceal  those  first  observed,  the 
existence  of  the  electron  in  the  cathodic  flux  remains 
the  essential  characteristic. 

The  electron  can  be  apprehended  in  the  cathodic 
ray  by  the  study  of  its  essential  properties;  and 
J.  J.  Thomson  gave  great  value  to  the  hypothesis 
by  his  measurements.  At  first  he  meant  to  deter- 
mine the  speed  of  the  cathode  rays  by  direct 
experiment,  and  by  observing,  in  a  revolving 
mirror,  the  relative  displacement  of  two  bands  due 
to  the  excitement  of  two  fluorescent  screens  placed 
at  different  distances  from  the  cathode.  But  he 
soon  perceived  that  the  effect  of  the  fluorescence  was 
not  instantaneous,  and  that  the  lapse  of  time  might 
form  a  great  source  of  error,  and  he  then  had 
recourse  to  indirect  methods.  It  is  possible,  by  a 
simple  calculation,  to  estimate  the  deviations  pro- 
duced on  the  rays  by  a  magnetic  and  an  electric  field 
respectively  as  a  function  of  the  speed  of  propaga- 
tion and  of  the  relation  of  the  charge  to  the  material 
mass  of  the  electron.  The  measurement  of  these 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    267 

deviations  will  then  permit  this  speed  and  this 
relation  to  be  ascertained. 

Other  processes  may  be  used  which  all  give 
the  same  two  quantities  by  two  suitably  chosen 
measurements.  Such  are  the  radius  of  the  curve 
.taken  by  the  trajectory  of  the  pencil  in  a  perpen- 
dicular magnetic  field  and  the  measure  of  the  fall  of 
potential  under  which  the  discharge  takes  place,  or 
the  measure  of  the  total  quantity  of  electricity 
carried  in  one  second  and  the  measure  of  the  calorific 
energy  which  may  be  given,  during  the  same  period, 
to  a  thermo-electric  junction.  The  results  agree  as 
well  as  can  be  expected,  having  regard  to  the 
difficulty  of  the  experiments ;  the  values  of  the 
speed  agree  also  with  those  which  Professor  Wiechert 
has  obtained  by  direct  measurement. 

The  speed  never  depends  on  the  nature  of  the  gas 
contained  in  the  Crookes  tube,  but  varies  with  the 
value  of  the  fall  of  potential  at  the  cathode.  It  is 
of  the  order  of  one  tenth  of  the  speed  of  light,  and  it 
may  rise  as  high  as  one  third.  The  cathode  particle 
therefore  goes  about  three  thousand  times  faster 
than  the  earth  in  its  orbit.  The  relation  is  also 
invariable,  even  when  the  substance  of  which  the 
cathode  is  formed  is  changed  or  one  gas  is  substituted 
for  another.  It  is,  on  the  average,  a  thousand  times 
greater  than  the  corresponding  relation  in  electrolysis. 
As  experiment  has  shown,  in  all  the  circumstances 
where  it  has  been  possible  to  effect  measurements, 


268    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

the  equality  of  the  charges  carried  by  all  corpuscules, 
ions,  atoms,  etc.,  we  ought  to  consider  that  the  charge 
of  the  electron  is  here,  again,  that  of  a  univalent  ion 
in  electrolysis,  and  therefore  that  its  mass  is  only  a 
small  fraction  of  that  of  the  atom  of  hydrogen, 
viz.,  of  the  order  of  about  a  thousandth  part.  This 
is  the  same  result  as  that  to  which  we  were  led  by 
the  study  of  flames. 

The  thorough  examination  of  the  cathode  radia- 
tion, then,  confirms  us  in  the  idea  that  every 
material  atom  can  be  dissociated  and  will  yield  an 
electron  much  smaller  than  itself — and  always  iden- 
tical whatever  the  matter  whence  it  comes, — the  res  t 
of  the  atom  remaining  charged  with  a  positive 
quantity  equal  and  contrary  to  that  borne  by  the 
electron.  In  the  present  case  these  positive  ions  are 
no  doubt  those  that  we  again  meet  with  in  the  canal 
rays.  Professor  Wien  has  shown  that  their  mass  is 
really,  in  fact,  of  the  order  of  the  mass  of  atoms. 
Although  they  are  all  formed  of  identical  electrons, 
there  may  be  various  cathode  rays,  because  the 
velocity  is  not  exactly  the  same  for  all  electrons. 
Thus  is  explained  the  fact  that  we  can  separate  them 
and  that  we  can  produce  a  sort  of  spectrum  by  the 
action  of  the  magnet,  or,  again,  as  M.  Deslandres  has 
shown  in  a  very  interesting  experiment,  by  that  of 
an  electrostatic  field.  This  also  probably  explains 
the  phenomena  studied  by  M.  Villard,  and  previously 
pointed  out. 


CATHODE    RAYS  AND  RADIOACTIVE  BODIES    269 

§  2.  RADIOACTIVE  SUBSTANCES 

Even  in  ordinary  conditions,  certain  substances 
called  radioactive  emit,  quite  outside  any  particular 
reaction,  radiations  complex  indeed,  but  which  pass 
through  fairly  thin  layers  of  minerals,  impress  photo- 
graphic plates,  excite  fluorescence,  and  ionize  gases. 
In  these  radiations  we  again  find  electrons  which 
thus  escape  spontaneously  from  radioactive  bodies. 

It  is  not  necessary  to  give  here  a  history  of  the 
discovery  of  radium,  for  every  one  knows  the 
admirable  researches  of  M.  and  Madame  Curie. 
But  subsequent  to  these  first  studies,  a  great  number 
of  facts  have  accumulated  for  the  last  six  years, 
among  which  some  people  find  themselves  a  little 
lost.  It  may,  perhaps,  not  be  useless  to  indicate  the 
essential  results  actually  obtained. 

The  researches  on  radioactive  substances  have 
their  starting-point  in  the  discovery  of  the  rays  of 
uranium  made  by  M.  Becquerel  in  1896.  As  early 
as  1867  Niepce  de  St  Victor  proved  that  salts  of 
uranium  impressed  photographic  plates  in  the  dark  ; 
but  at  that  time  the  phenomenon  could  only  pass  for 
a  singularity  attributable  to  phosphorescence,  and 
the  valuable  remarks  of  Niepce  fell  into  oblivion. 
M.  Becquerel  established,  after  some  hesitations 
natural  in  the  face  of  phenomena  which  seemed  so 
contrary  to  accepted  ideas,  that  the  radiating  pro- 
perty was  absolutely  independent  of  phosphorescence, 


270    THE   NEW  PHYSICS  AND  ITS   EVOLUTION 

that  all  the  salts  of  uranium,  even  the  uranous 
salts  which  are  not  phosphorescent,  give  similar 
radiant  effects,  and  that  these  phenomena  corre- 
spond to  a  continuous  emission  of  energy,  but  do 
not  seem  to  be  the  result  of  a  storage  of  energy 
under  the  influence  of  some  external  radiation. 
Spontaneous  and  constant,  the  radiation  is  insensible 
to  variations  of  temperature  and  light. 

The  nature  of  these  radiations  was  not  imme- 
diately understood,1  and  their  properties  seemed  con- 
tradictory. This  was  because  we  were  not  dealing 
with  a  single  category  of  rays.  But  amongst  all  the 
effects  there  is  one  which  constitutes  for  the  radia- 
tions taken  as  a  whole,  a  veritable  process  for  the 
measurement  of  radioactivity.  This  is  their  ionizing 
action  on  gases.  A  very  complete  study  of  the 
conductivity  of  air  under  the  influence  of  rays  of 
uranium  has  been  made  by  various  physicists,  par- 
ticularly by  Professor  Eutherford,  and  has  shown 
that  the  laws  of  the  phenomenon  are  the  same  as 
those  of  the  ionization  due  to  the  action  of  the 
Rontgen  rays. 

It  was  natural  to  ask  one's  self  if  the  property  dis- 
covered in  salts  of  uranium  was  peculiar  to  this  body, 

1  In  his  work  on  I? Evolution  de  la  Matiere,  M.  Gustave  Le 
Bon  recalls  that  in  1897  he  published  several  notes  in  the 
Academic  des  Sciences,  in  which  he  asserted  that  the  properties 
of  uranium  were  only  a  particular  case  of  a  very  general  law, 
and  that  the  radiations  emitted  did  not  polarize,  and  were 
akin  by  their  properties  to  the  X  rays. 


CATHODE  KAYS  AND   KADIOACTIVE  BODIES    271 

or  if  it  were  not,  to  a  more  or  less  degree,  a  general 
property  of  matter.  Madame  Curie  and  M.  Schmidt, 
independently  of  each  other,  made  systematic 
researches  in  order  to  solve  the  question;  various 
compounds  of  nearly  all  the  simple  bodies  at  present 
known  were  thus  passed  in  review,  and  it  was 
established  that  radioactivity  was  particularly  per- 
ceptible in  the  compounds  of  uranium  and  thorium, 
and  that  it  was  an  atomic  property  linked  to  the 
matter  endowed  with  it,  and  following  it  in  all  its 
combinations.  In  the  course  of  her  researches 
Madame  Curie  observed  that  certain  pitchblendes 
(oxide  of  uranium  ore,  containing  also  barium,  bis- 
muth, etc.)  were  four  times  more  active  (activity  being 
measured  by  the  phenomenon  of  the  ionization  of 
the  air)  than  metallic  uranium.  Now,  no  compound 
containing  any  other  active  metal  than  uranium 
or  thorium  ought  to  show  itself  more  active  than 
those  metals  themselves,  since  the  property  belongs 
to  their  atoms.  It  seemed,  therefore,  probable  that 
there  existed  in  pitchblendes  some  substance  yet 
unknown,  in  small  quantities  and  more  radioactive 
than  uranium. 

M.  and  Madame  Curie  then  commenced  those 
celebrated  experiments  which  brought  them  to  the 
discovery  of  radium.  Their  method  of  research  has 
been  justly  compared  in  originality  and  importance 
to  the  process  of  spectrum  analysis.  To  isolate  a 
radioactive  substance,  the  first  thing  is  to  measure 


272    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

the  activity  of  a  certain  compound  suspected  of 
containing  this  substance,  and  this  compound  is 
chemically  separated.  We  then  again  take  in  hand 
all  the  products  obtained,  and  by  measuring  their 
activity  anew,  it  is  ascertained  whether  the  substance 
sought  for  has  remained  in  one  of  these  products,  or 
is  divided  among  them,  and  if  so,  in  what  proportion. 
The  spectroscopic  reaction  which  we  may  use  in  the 
course  of  this  separation  is  a  thousand  times  less 
sensitive  than  observation  of  the  activity  by  means 
of  the  electrometer. 

Though  the  principle  on  which  the  operation  of 
the  concentration  of  the  radium  rests  is  admirable 
in  its  simplicity,  its  application  is  nevertheless  very 
laborious.  Tons  of  uranium  residues  have  to  be 
treated  in  order  to  obtain  a  few  decigrammes  of 
pure  salts  of  radium.  Radium  is  characterised  by 
a  special  spectrum,  and  its  atomic  weight,  as  deter- 
mined by  Madame  Curie,  is  225 ;  it  is  consequently 
the  higher  homologue  of  barium  in  one  of  the  groups 
of  Mendeleef.  Salts  of  radium  have  in  general  the 
same  chemical  properties  as  the  corresponding  salts 
of  barium,  but  are  distinguished  from  them  by  the 
differences  of  solubility  which  allow  of  their  separa- 
tion, and  by  their  enormous  activity,  which  is  about  a 
hundred  thousand  times  greater  than  that  of  uranium. 

Eadium  produces  various  chemical  and  some  very 
intense  physiological  reactions.  Its  salts  are  luminous 
in  the  dark,  but  this  luminosity,  at  first  very  bright, 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    273 

gradually  diminishes  as  the  salts  get  older.  We  have 
here  to  do  with  a  secondary  reaction  correlative  to 
the  production  of  the  emanation,  after  which  radium 
undergoes  the  transformations  which  will  be  studied 
later  on. 

The  method  of  analysis  founded  by  M.  and  Madame 
Curie  has  enabled  other  bodies  presenting  sensible 
radioactivity  to  be  discovered.  The  alkaline  metals 
appear  to  possess  this  property  in  a  slight  degree. 
Eecently  fallen  snow  and  mineral  waters  manifest 
marked  action.  The  phenomenon  may  often  be 
due,  however,  to  a  radioactivity  induced  by  radia- 
tions already  existing  in  the  atmosphere.  But  this 
radioactivity  hardly  attains  the  ten-thousandth 
part  of  that  presented  by  uranium,  or  the  ten- 
millionth  of  that  appertaining  to  radium. 

Two  other  bodies,  polonium  and  actinium,  the  one 
characterised  by  the  special  nature  of  tl»  radiations 
it  emits  and  the  other  by  a  particular  spectrum, 
seem  likewise  to  exist  in  pitchblende.  These 
chemical  properties  have  not  yet  been  perfectly 
denned ;  thus  M.  Debierne,  who  discovered  actinium, 
has  been  able  to  note  the  active  property  which  seems 
to  belong  to  it,  sometimes  in  lanthanum,  sometimes 
in  neodynium.1  It  is  proved  that  all  extremely  radio- 

1  Polonium  lias  now  been  shown  to  be  no  new  element,  but 
one  of  the  transformation  products  of  radium.  Radium  itself 
is  also  thought  to  be  derived  in  some  manner,  not  yet  ascer- 
tained, from  uranium.  The  same  is  the  case  with  actinium, 

18 


274    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

active  bodies  are  the  seat  of  incessant  transforma- 
tions, and  even  now  we  cannot  state  the  conditions 
under  which  they  present  themselves  in  a  strictly 
determined  form. 

§  3.  THE  KADIATION  OF  THE  KADIOACTIVE  BODIES 
AND  THE  EMANATION 

To  acquire  exact  notions  as  to  the  nature  of 
the  rays  emitted  by  the  radioactive  bodies,  it  was 
necessary  to  try  to  cause  magnetic  or  electric  forces 
to  act  on  them  so  as  to  see  whether  they  behaved  in 
the  same  way  as  light  and  the  X  rays,  or  whether 
like  the  cathode  rays  they  were  deviated  by  a 
magnetic  field.  This  work  was  effected  by  Professor 
Giesel,  then  by  M.  Becquerel,  Professor  Eutherford, 
and  by  many  other  experimenters  after  them.  All 
the  methods  which  have  already  been  mentioned  in 
principle  have  been  employed  in  order  to  discover 
whether  they  were  electrified,  and,  if  so,  by  electricity 
of  what  sign,  to  measure  their  speed,  and  to  ascertain 
their  degree  of  penetration. 

The  general  result  has  been  to  distinguish  three 
sorts  of  radiations,  designated  by  the  letters  a,  /3,  y. 

The  a  rays  are  positively  charged,  and  are  pro- 
jected at  a  speed  which  may  attain  the  tenth  of  that 

which  is  said  to  come  in  the  long  run  from  uranium,  but  not 
so  directly  as  does  radium.  All  this  is  described  in  Professor 
Rutherford's  Radioactive  Transformations  (London,  1906). — 
ED. 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    275 

of  light ;  M.  H.  Becquerel  has  shown  by  the  aid  of 
photography  that  they  are  deviated  by  a  magnet, 
and  Professor  Eutherford  has,  on  his  side,  studied 
this  deviation  by  the  electrical  method.  The  relation 
of  the  charge  to  the  mass  is,  in  the  case  of  these 
rays,  of  the  same  order  as  in  that  of  the  ions  of 
electrolysis.  They  may  therefore  be  considered  as 
exactly  analogous  to  the  canal  rays  of  Goldstein,  and 
we  may  attribute  them  to  a  material  transport  of 
corpuscles  of  the  magnitude  of  atoms.  The  rela- 
tively considerable  size  of  these  corpuscles  renders 
them  very  absorbable.  A  flight  of  a  few  millimetres 
in  a  gas  suffices  to  reduce  their  number  by  one-half. 
They  have  great  ionizing  power. 

The  /3  rays  are  on  all  points  similar  to  the  cathode 
rays ;  they  are,  as  M.  and  Madame  Curie  have  shown, 
negatively  charged,  and  the  charge  they  carry  is 
always  the  same.  Their  size  is  that  of  the  electrons, 
and  their  velocity  is  generally  greater  than  that  of 
the  cathode  rays,  while  it  may  become  almost  that 
of  light.  They  have  about  a  hundred  times  less 
ionizing  power  than  the  a  rays. 

The  y  rays  were  discovered  by  M.  Villard.1    They 

1  This  is  admitted  by  Professor  Rutherford  (Radio- Activity, 
Camb.,  1904,  p.  141)  and  Professor  Soddy  (Radio- Activity, 
London,  1904,  p.  66).  Neither  Mr  Whetham,  in  his  Recent 
Development  of  Physical  Science  (London,  1904)  nor  the  Hon. 
R.  J.  Strutt  in  The  Becquerel  Rays  (London,  same  date),  both 
of  whom  deal  with  the  historical  side  of  the  subject,  seem  to- 
have  noticed  the  fact. — ED. 


276     THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

may  be  compared  to  the  X  rays ;  like  the  latter,  they 
are  not  deviated  by  the  magnetic  field,  and  are  also 
extremely  penetrating.  A  strip  of  aluminium  five 
millimetres  thick  will  stop  the  other  kinds,  but  will 
allow  them  to  pass.  On  the  other  hand,  their  ionizing 
power  is  10,000  times  less  than  that  of  the  a  rays. 

To  these  radiations  there  sometimes  are  added 
in  the  course  of  experiments  secondary  radiations 
analogous  to  those  of  M.  Sagnac,  and  produced 
when  the  a,  /3,  or  y  rays  meet  various  substances. 
This  complication  has  often  led  to  some  errors  of 
observation. 

Phosphorescence  and  fluorescence  seem  especially 
to  result  from  the  a  and  /3  rays,  particularly  from 
the  a  rays,  to  which  belongs  the  most  important 
part  of  the  total  energy  of  the  radiation.  Sir  W. 
Crookes  has  invented  a  curious  little  apparatus,  the 
spinthariscope,  which  enables  us  to  examine  the 
phosphorescence  of  the  blende  excited  by  these 
rays.  By  means  of  a  magnifying  glass,  a  screen 
covered  with  sulphide  of  zinc  is  kept  under  observa- 
tion, and  in  front  of  it  is  disposed,  at  a  distance  of 
about  half  a  millimetre,  a  fragment  of  some  salt  of 
radium.  We  then  perceive  multitudes  of  brilliant 
points  on  the  screen,  which  appear  and  at  once 
disappear,  producing  a  scintillating  effect.  It  seems 
probable  that  every  particle  falling  on  the  screen 
produces  by  its  impact  a  disturbance  in  the  neigh- 
bouring region,  and  it  is  this  disturbance  which  the 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    277 

eye  perceives  as*  a  luminous  point.  Thus,  says  Sir  W. 
Crookes,  each  drop  of  rain  falling  on  the  surface  of 
still  water  is  not  perceived  as  a  drop  of  rain,  but  by 
reason  of  the  slight  splash  which  it  causes  at  the 
moment  of  impact,  and  which  is  manifested  by 
ridges  and  waves  spreading  themselves  in  circles. 

The  various  radioactive  substances  do  not  all 
give  radiations  of  identical  constitution.  Kadium 
and  thorium  possess  in  somewhat  large  proportions 
the  three  kinds  of  rays,  and  it  is  the  same  with 
actinium.  Polonium  contains  especially  a  rays  and 
a  few  y  rays.1  In  the  case  of  uranium,  the  a  rays 
have  extremely  slight  penetrating  power,  and  cannot 
even  impress  photographic  plates.  But  the  widest 
difference  between  the  substances  proceeds  from 
the  emanation.  Kadium,  in  addition  to  the  three 
groups  of  rays  a,  ft,  and  y,  disengages  continuously 
an  extremely  subtle  emanation,  seemingly  almost 
imponderable,  but  which  may  be,  for  many  reasons, 
looked  upon  as  a  vapour  of  which  the  elastic  force 
is  extremely  feeble. 

M.  and  Madame  Curie  discovered  as  early  as 
1899  that  every  substance  placed  in  the  neighbour- 
hood of  radium,  itself  acquired  a  radioactivity  which 
persisted  for  several  hours  after  the  removal  of  the 
radium.  This  induced  radioactivity  seems  to  be 

1  It  has  now  been  shown  that  polonium  when  freshly 
separated  emits  ft  rays  also ;  see  Dr  Logeman's  paper  in  Pro- 
ceedings of  the  Royal  Society,  A.,  6th  September  1906. — ED. 


278    THE  NEW   PHYSICS  AND  ITS  EVOLUTION 

carried  to  other  bodies  by  the  intermediary  of  a 
gas.  It  goes  round  obstacles,  but  there  must  exist 
between  the  radium  and  the  substance  a  free  and 
continuous  space  for  the  activation  to  take  place ;  it 
cannot,  for  instance,  do  so  through  a  wall  of  glass. 

In  the  case  of  compounds  of  thorium  Professor 
Kutherford  discovered  a  similar  phenomenon ;  since 
then,  various  physicists,  Professor  Soddy,  Miss  Brooks, 
Miss  Gates,  M.  Danne,  and  others,  have  studied  the 
properties  of  these  emanations. 

The  substance  emanated  can  neither  be  weighed 
nor  can  its  elastic  force  be  ascertained;  but  its 
transformations  may  be  followed,  as  it  is  luminous, 
and  it  is  even  more  certainly  characterised  by  its 
essential  property,  i.e.  its  radioactivity.  We  also 
see  that  it  can  be  decanted  like  a  gas,  that  it 
will  divide  itself  between  two  tubes  of  different 
capacity  in  obedience  to  the  law  of  Mariotte,  and 
will  condense  in  a  refrigerated  tube  in  accordance 
with  the  principle  of  Watt,  while  it  even  complies 
with  the  law  of  Gay-Lussac. 

The  activity  of  the  emanation  vanishes  quickly, 
and  at  the  end  of  four  days  it  has  diminished  by  one- 
half.  If  a  salt  of  radium  is  heated,  the  emanation 
becomes  more  abundant,  and  the  residue,  which, 
however,  does  not  sensibly  diminish  in  weight,  will 
have  lost  all  its  radioactivity,  and  will  only  recover  it 
by  degrees.  Professor  Eutherford,  notwithstanding 
many  different  attempts,  has  been  unable  to  make 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    279 

• 

this  emanation  enter  into  any  chemical  reaction. 
If  it  be  a  gaseous  body,  it  must  form  part  of  the 
argon  group,  and,  like  its  other  members,  be 
perfectly  inert. 

By  studying  the  spectrum  of  the  gas  disengaged 
by  a  solution  of  salt  of  -radium,  Sir  William  Eamsay 
and  Professor  Soddy  remarked  that  when  the  gas  is 
radioactive  there  are  first  obtained  rays  of  gases 
belonging  to  the  argon  family,  then  by  degrees,  as 
the  activity  disappears,  the  spectrum  slowly  changes, 
and  finally  presents  the  characteristic  aspect  of 
helium. 

We  know  that  the  existence  of  this  gas  was  first 
discovered  by  spectrum  analysis  in  the  sun.  Later 
its  presence  was  noted  in  our  atmosphere,  and  in  a 
few  minerals  which  happen  to  be  the  very  ones  from 
which  radium  has  been  obtained.  It  might  therefore 
have  been  the  case  that  it  pre-existed  in  the  gases 
extracted  from  radium  ;  but  a  remarkable  experiment 
by  M.  Curie  and  Sir  James  Dewar  seems  to  show 
convincingly  that  this  cannot  be  so.  The  spec- 
trum of  helium  never  appears  at  first  in  the  gas 
proceeding  from  pure  bromide  of  radium ;  but  it 
shows  itself,  on  the  other  hand,  very  distinctly,  after 
the  radioactive  transformations  undergone  by  the  salt. 

All  these  strange  phenomena  suggest  bold  hypo- 
theses, but  to  construct  them  with  any  solidity  they 
must  be  supported  by  the  greatest  possible  number 
of  facts.  Before  admitting  a  definite  explanation  of 


280    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

• 

the  phenomena  which  have  their  seat  in  the  curious 
substances  discovered  by  them,  M.  and  Madame 
Curie  considered,  with  a  great  deal  of  reason,  that 
they  ought  first  to  enrich  our  knowledge  with  the 
exact  and  precise  facts  relating  to  these  bodies  and 
to  the  effects  produced  by  the  radiations  they  emit. 

Thus  M.  Curie  particularly  set  himself  to  study 
the  manner  in  which  the  radioactivity  of  the 
emanation  is  dissipated,  and  the  radioactivity  that 
this  emanation  can  induce  on  all  bodies.  The  radio- 
activity of  the  emanation  diminishes  in  accordance 
with  an  exponential  law.  The  constant  of  time 
which  characterises  this  decrease  is  easily  and 
exactly  determined,  and  has  a  fixed  value,  inde- 
pendent of  the  conditions  of  the  experiment  as  well 
as  of  the  nature  of  the  gas  which  is  in  contact  with 
the  radium  and  becomes  charged  with  the  emanation. 
The  regularity  of  the  phenomenon  is  so  great  that  it 
can  be  used  to  measure  time :  in  3985  seconds l  the 
activity  is  always  reduced  one-half. 

Eadioactivity  induced  on  any  body  which  has 
been  for  a  long  time  in  presence  of  a  salt  of  radium 
disappears  more  rapidly.  The  phenomenon  appears, 
moreover,  more  complex,  arid  the  formula  which 
expresses  the  manner  in  which  the  activity  dim- 
inishes must  contain  two  exponentials.  To  find  it 
theoretically  we  have  to  imagine  that  the  emana- 
tion first  deposits  on  the  body  in  question  a  substance 

1  According  to  Professor  Rutherford,  in  3-77  days. — ED. 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    281 

which  is  destroyed  in  giving  birth  to  a  second,  this 
latter  disappearing  in  its  turn  by  generating  a  third. 
The  initial  and  final  substances  would  be  radioactive, 
but  the  intermediary  one,  not.  If,  moreover,  the 
bodies  acted  on  are  brought  to  a  temperature  of  over 
700°,  they  appear  to  lose  by  volatilisation  certain 
substances  condensed  in  them,  and  at  the  same  time 
their  activity  disappears. 

The  other  radioactive  bodies  behave  in  a 
similar  way.  Bodies  which  contain  actinium  are 
particularly  rich  in  emanations.  Uranium,  on  the 
contrary,  has  none.1  This  body,  nevertheless,  is  the 
seat  of  transformations  comparable  to  those  which 
the  study  of  emanations  reveals  in  radium ;  Sir 
W.  Crookes  has  separated  from  uranium  a  matter 
which  is  now  called  uranium  X.  This  matter  is  at 
first  much  more  active  than  its  parent,  but  its  activity 
diminishes  rapidly,  while  the  ordinary  uranium, 
which  at  the  time  of  the  separation  loses  its  activity, 
regains  it  by  degrees.  In  the  same  way,  Professors 
Eutherford  and  Soddy  have  discovered  a  so-called 
thorium  X  to  be  the  stage  through  which  ordinary 
thorium  has  to  pass  in  order  to  produce  its 
emanation.2 

1  Professor  Rutherford  has  lately  stated  that  uranium  may 
possibly  produce  an  emanation,  but  that  its  rate  of  decay  must 
be  too  swift  for  its   presence  to  be  verified  (see  Radioactive 
Transformations,  p.  161). — ED. 

2  An  actinium  X  was  also  discovered  by  Professor  Giesel 
(Jahrbuch  d.  Radioaktivitat,  i.  p.  358,  1904).     Since  the  above 


282    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

It  is  not  possible  to  give  a  complete  table  which 
should,  as  it  were,  represent  the  genealogical  tree  of 
the  various  radioactive  substances.  Several  authors 
have  endeavoured  to  do  so,  but  in  a  premature 
manner;  all  the  affiliations  are  not  at  the  present 
time  yet  perfectly  known,  and  it  will  no  doubt  be 
acknowledged  some  day  that  identical  states  have 
been  described  under  different  names.1 

§  4.  THE  DlSAGGREGATlON  OF  MATTER  AND 
ATOMIC  ENERGY 

In  spite  of  uncertainties  which  are  not  yet  entirely 
removed,  it  cannot  be  denied  that  many  experiments 
render  it  probable  that  in  radioactive  bodies  we 
find  ourselves  witnessing  veritable  transformations 
of  matter. 

Professor  Eutherford,  Professor  Soddy,  and  several 
other  physicists,  have  come  to  regard  these  phenomena 
in  the  following  way.  A  radioactive  body  is  com- 
posed of  atoms  which  have  little  stability,  and  are 
able  to  detach  themselves  spontaneously  from  the 
parent  substance,  and  at  the  same  time  to  divide 
themselves  into  two  essential  component  parts,  the 
negative  electron  and  its  residue  the  positive  ion. 

was  written,  another  product  has  been  found  to  intervene 
between  the  X  substance  and  the  emanation  in  the  case  of 
actinium  and  thorium.  They  have  been  named  radio-actinium 
and  radio-thorium  respectively. — ED. 

1  Such  a  table  is  given  on  p.  169  of  Rutherford's  Radio- 
active Transformations. — ED. 


CATHODE  RA7S  AND  RADIOACTIVE  BODIES    283 

The   first-named  constitutes  the  /3,  and  the  second 
the  a  rays. 

The  emanation  is  certainly  composed  of  «  ions 
with  a  few  molecules  agglomerated  round  them. 
Professor  Eutherford  has,  in  fact,  demonstrated  that 
the  emanation  is  charged  with  positive  electricity; 
and  this  emanation  may,  in  turn,  be  destroyed  by 
giving  birth  to  new  bodies. 

'  After  the  loss  of  the  atoms  which  are  carried  off 
by  the  radiation,  the  remainder  of  the  body  acquires 
new  properties,  but  it  may  still  be  radioactive,  and 
again  lose  atoms.  The  various  stages  that  we  meet 
with  in  the  evolution  of  the  radioactive  substance 
or  of  its  emanation,  correspond  to  the  various  degrees 
of  atomic  disaggregation.  Professors  Eutherford  and 
Soddy  have  described  them  clearly  in  the  case  of 
uranium  and  radium.  As  regards  thorium  the 
results  are  less  satisfactory.  The  evolution  should 
continue  until  a  stable  atomic  condition  is  finally 
reached,  which,  because  of  this  stability,  is  no  longer 
radioactive.  Thus,  for  instance,  radium  would  finally 
be  transformed  into  helium.1 

It  is  possible,  by  considerations  analogous  to  those 

1  This  opinion,  no  doubt  formed  when  Sir  William  Ramsay's 
discovery  of  the  formation  of  helium  from  the  radium  emana- 
tion was  first  made  known,  is  now  less  tenable.  The  latest 
theory  is  that  the  a  particle  is  in  fact  an  atom  of  helium,  and 
that  the  final  transformation  product  of  radium  and  the  other 
radioactive  substances  is  lead.  Cf.  Rutherford,  op.  cit.  passim. 
—ED. 


284    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

set  forth  above  in  other  cases,  to  arrive  at  an  idea 
of  the  total  number  of  particles  per  second  expelled 
by  one  gramme  of  radium ;  Professor  Eutherford 
in  his  most  recent  evaluation  finds  that  this  number 
approaches  2*5  x  1 011.1  By  calculating  from  the  atomic 
weight  the  number  of  atoms  probably  contained  in 
this  gramme  of  radium,  and  supposing  each  particle 
liberated  to  correspond  to  the  destruction  of  one 
atom,  it  is  found  that  one  half  of  the  radium  should 
disappear  in  1280  years;2  and  from  this  we  may 
conceive  that  it  has  not  yet  been  possible  to  discover 
any  sensible  loss  of  weight.  Sir  W.  Eamsay  and 
Professor  Soddy  attained  a  like  result  by  endeavour- 
ing to  estimate  the  mass  of  the  emanation  by  the 
quantity  of  helium  produced. 

If  radium  transforms  itself  in  such  a  way  that  its 
activity  does  not  persist  throughout  the  ages,  it 
loses  little  by  little  the  provision  of  energy  it  had 
in  the  beginning,  and  its  properties  furnish  no  valid 
argument  to  oppose  to  the  principle  of  the  conserva- 
tion of  energy.  To  put  everything  right,  we  have 
only  to  recognise  that  radium  possessed  in  the 
potential  state  at  its  formation  a  finite  quantity  of 

1  See    Radioactive     Transformations    (p.    251).      Professor 
Kutherf ord  says  that  "  each  of  the  a  ray  products  present  in 
one  gram  of  radium  product  (sic)  expels  6*2  x  1010  o  particles 
per  second."    He  also  remarks  on  "  the  experimental  difficulty 
of  accurately  determining  the  number  of  o  particles  expelled 
from  radium  per  second." — ED. 

2  See  Rutherford,  op.  cit.  p.  150. — ED. 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    285 

energy  which  is  consumed  little  by  little.  In  the 
same  manner,  a  chemical  system  composed,  for 
instance,  of  zinc  and  sulphuric  acid,  also  contains 
in  the  potential  state  energy  which,  if  we  retard  the 
reaction  by  any  suitable  arrangement — such  as  by 
amalgamating  the  zinc  and  by  constituting  with 
its  elements  a  battery  which  we  cause  to  act  on  a 
resistance — may  be  made  to  exhaust  itself  as  slowly 
as  one  may  desire. 

There  can,  therefore,  be  nothing  in  any  way  sur- 
prising in  the  fact  that  a  combination  which,  like 
the  atomic  combination  of  radium,  is  not  stable — since 
it  disaggregates  itself, — is  capable  of  spontaneously 
liberating  energy,  but  what  may  be  a  little  astonish- 
ing, at  first  sight,  is  the  considerable  amount  of  this 
energy. 

M.  Curie  has  calculated  directly,  by  the  aid  of  the 
calorimeter,  the  quantity  of  energy  liberated,  measur- 
ing it  entirely  in  the  form  of  heat.  The  disengage- 
ment of  heat  accounted  for  in  a  grain  of  radium 
is  uniform,  and  amounts  to  100  calories  per  hour. 
It  must  therefore  be  admitted  that  an  atom  of 
radium,  in  disaggregating  itself,  liberates  30,000 
times  more  energy  than  a  molecule  of  hydrogen 
when  the  latter  combines  with  an  atom  of  oxygen 
to  form  a  molecule  of  water. 

We  may  ask  ourselves  how  the  atomic  edifice  of 
the  active  body  can  be  constructed,  to  contain  so 
great  a  provision  of  energy.  We  will  remark  that 


286    THE  NEW  PHYSICS   AND  ITS  EVOLUTION 

such  a  question  might  be  asked  concerning  cases 
known  from  the  most  remote  antiquity,  like  that  of 
the  chemical  systems,  without  any  satisfactory  answer 
ever  being  given.  This  failure  surprises  no  one,  for 
we  get  used  to  everything — even  to  defeat. 

When  we  come  to  deal  with  a  new  problem  we 
have  really  no  right  to  show  ourselves  more  exacting ; 
yet  there  are  found  persons  who  refuse  to  admit  the 
hypothesis  of  the  atomic  disaggregation  of  radium 
because  they  cannot  have  set  before  them  a  detailed 
plan  of  that  complex  whole  known  to  us  as  an  atom. 

The  most  natural  idea  is  perhaps  the  one  suggested 
by  comparison  with  those  astronomical  phenomena 
where  our  observation  most  readily  allows  us  to  com- 
prehend the  laws  of  motion.  It  corresponds  likewise 
to  the  tendency  ever  present  in  the  mind  of  man,  to 
compare  the  infinitely  small  with  the  infinitely  great. 
The  atom  may  be  regarded  as  a  sort  of  solar  system 
in  which  electrons  in  considerable  numbers  gravitate 
round  the  sun  formed  by  the  positive  ion.  It  may 
happen  that  certain  of  these  electrons  are  no  longer 
retained  in  their  orbit  by  the  electric  attraction  of  the 
rest  of  the  atom,  and  may  be  projected  from  it  like 
a  small  planet  or  comet  which  escapes  towards  the 
stellar  spaces.  The  phenomena  of  the  emission  of 
light  compels  us  to  think  that  the  corpuscles  revolve 
round  the  nucleus  with  extreme  velocities,  or  at 
the  rate  of  thousands  of  billions  of  evolutions 
per  second.  It  is  easy  to  conceive  from  this  that, 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES    287 

notwithstanding  its  lightness,  an  atom  thus  con- 
stituted may  possess  an  enormous  energy.1 

Other  authors  imagine  that  the  energy  of  the 
corpuscles  is  principally  due  to  the  extremely  rapid 
rotations  of  those  elements  on  their  own  axes.  Lord 
Kelvin  lately  drew  up  on  another  model  the  plan  of 
a  radioactive  atom  capable  of  ejecting  an  electron 
with  a  considerable  vis  viva.  He  supposes  a  spherical 
atom  formed  of  concentric  layers  of  positive  and 
negative  electricity  disposed  in  such  a  way  that  its 
external  action  is  null,  and  that,  nevertheless,  the 
force  emanated  from  the  centre  may  be  repellent  for 
certain  values  when  the  electron  is  within  it. 

The  most  prudent  physicists  and  those  most 
respectful  to  established  principles  may,  without 
any  scruples,  admit  the  explanation  of  the  radio- 
activity of  radium  by  a  dislocation  of  its  molecular 
edifice.  The  matter  of  which  it  is  constituted 
evolves  from  an  admittedly  unstable  initial  state 
to  another  stable  one.  It  is,  in  a  way,  a  slow  allo- 
tropic  transformation  which  takes  place  by  means 
of  a  mechanism  regarding  which,  in  short,  we  have 
no  more  information  than  we  have  regarding  other 
analogous  transformations.  The  only  astonishment 
we  can  legitimately  feel  is  derived  from  the  thought 

1  This  view  of  the  case  has  been  made  very  clear  by  M. 
Gustave  le  Bon  in  L' Evolution  de  la  Matiere  (Paris,  1906). 
See  especially  pp.  36-52,  where  the  amount  of  the  supposed 
intra-atomic  energy  is  calculated. — ED. 


288    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

that  we  are  suddenly  and  deeply  penetrating  to  the 
very  heart  of  things. 

But  those  persons  who  have  a  little  more  hardi- 
hood do  not  easily  resist  the  temptation  of  forming 
daring  generalisations.  Thus  it  will  occur  to  some 
that  this  property,  already  discovered  in  many  sub- 
stances where  it  exists  in  more  or  less  striking 
degree,  is,  with  differences  of  intensity,  common  to 
all  bodies,  and  that  we  are  thus  confronted  by  a 
phenomenon  derived  from  an  essential  quality  of 
matter.  Quite  recently,  Professor  Eutherford  has 
demonstrated  in  a  fine  series  of  experiments  that 
the  a  particles  of  radium  cease  to  ionize  gases  when 
they  are  made  to  lose  their  velocity,  but  that  they  do 
not  on  that  account  cease  to  exist.  It  may  follow 
that  many  bodies  emit  similar  particles  without  being 
easily  perceived  to  do  so  ;  since  the  electric  action,  by 
which  this  phenomenon  of  radioactivity  is  generally 
manifested,  would,  in  this  case,  be  but  very  weak. 

If  we  thus  believe  radioactivity  to  be  an  absol- 
utely general  phenomenon,  we  find  ourselves  face  to 
face  with  a  new  problem.  The  transformation  of 
radioactive  bodies  can  no  longer  be  assimilated  to 
allotropic  transformations,  since  thus  no  final  form 
could  ever  be  attained,  and  the  disaggregation  would 
continue  indefinitely  up  to  the  complete  dislocation 
of  the  atom.1  The  phenomenon  might,  it  is  true, 

1  This  is  the  main  contention  of  M.  Gustave  Le  Bon  in 
his  work  last  quoted. — ED. 


CATHODE  RAYS  AND  RADIOACTIVE  BODIES     289 

have  a  duration  of  perhaps  thousands  of  millions  of 
centuries,  but  this  duration  is  but  a  minute  in  the 
infinity  of  time,  and  matters  little.  Our  habits  of 
mind,  if  we  adopt  such  a  conception,  will  be  none 
the  less  very  deeply  disturbed.  We  shall  have  to 
abandon  the  idea  so  instinctively  dear  to  us  that 
matter  is  the  most  stable  thing  in  the  universe,  and 
to  admit,  on  the  contrary,  that  all  bodies  whatever 
are  a  kind  of  explosive  decomposing  with  extreme 
slowness.  There  is  in  this,  whatever  may  have  been 
said,  nothing  contrary  to  any  of  the  principles  on 
which  the  science  of  energetics  rests ;  but  an  hypo- 
thesis of  this  nature  carries  with  it  consequences 
which  ought  in  the  highest  degree  to  interest  the 
philosopher,  and  we  all  know  with  what  alluring 
boldness  M.  Gustave  Le  Bon  has  developed  all 
these  consequences  in  his  work  on  the  evolution  of 
matter.1 

There  is  hardly  a  physicist  who  does  not  at  the 
present  day  adopt  in  one  shape  or  another  the 
ballistic  hypothesis.  All  new  facts  are  co-ordinated 
so  happily  by  it,  that  it  more  and  more  satisfies 
our  minds ;  but  it  cannot  be  asserted  that  it  forces 
itself  on  our  convictions  with  irresistible  weight. 
Another  point  of  view  appeared  more  plausible 
and  simple  at  the  outset,  when  there  seemed  reason 
to  consider  the  energy  radiated  by  radioactive 
bodies  as  inexhaustible.  It  was  thought  that  the 
1  See  last  note.— ED. 

19 


290    THE   NEW  PHYSICS   AND   ITS   EVOLUTION 

source  of  this  energy  was  to  be  looked  for  without 
the  atom,  and  this  idea  may  perfectly  well  be  main- 
tained at  the  present  day. 

Eadium  on  this  hypothesis  must  be  considered 
as  a  transformer  borrowing  energy  from  the 
external  medium  and  returning  it  in  the  form  of 
radiation.  It  is  not  impossible,  even,  to  admit  that 
the  energy  which  the  atom  of  radium  withdraws 
from  the  surrounding  medium  may  serve  to  keep 
up,  not  only  the  heat  emitted  and  its  complex 
radiation,  but  also  the  dissociation,  supposed  to  be 
endothermic,  of  this  atom.  Such  seems  to  be  the 
idea  of  M.  Debierne  and  also  of  M.  Sagnac.  It  does 
not  seem  to  accord  with  the  experiments  that  this 
borrowed  energy  can  be  a  part  of  the  heat  of  the 
ambient  medium ;  and,  indeed,  such  a  phenomenon 
would  be  contrary  to  the  principle  of  Carnot  if  we 
wished  (though  we  have  seen  how  disputable  is 
this  extension)  to  extend  this  principle  to  the 
phenomena  which  are  produced  in  the  very  bosom 
of  the  atom. 

We  may  also  address  ourselves  to  a  more  noble 
form  of  energy,  and  ask  ourselves  whether  we  are 
not,  for  the  first  time,  in  presence  of  a  transformation 
of  gravitational  energy.  It  may  be  singular,  but  it 
is  not  absurd,  to  suppose  that  the  unit  of  mass  of 
radium  is  not  attached  to  the  earth  with  the  same  in- 
tensity as  an  inert  body.  M.  Sagnac  has  commenced 
some  experiments,  as  yet  unpublished,  in  order  to 


CATHODE  KAYS  AND  KADIO  ACTIVE  BODIES      291 

study  the  laws  of  the  fall  of  a  fragment  of  radium. 
They  are  necessarily  very  delicate,  and  the  energetic 
and  ingenious  physicist  has  not  yet  succeeded  in 
finishing  them.1  Let  us  suppose  that  he  succeeds 
in  demonstrating  that  the  intensity  of  gravity  is 
less  for  radium  than  for  the  platinum  or  the  copper 
of  which  the  pendulums  used  to  illustrate  the 
law  of  Newton  are  generally  made;  it  would 
then  be  possible  still  to  think  that  the  laws  of 
universal  attraction  are  perfectly  exact  as  regards 
the  stars,  and  that  ponderability  is  really  a  par- 
ticular case  of  universal  attraction,  while  in  the 
case  of  radioactive  bodies  part  of  the  gravitational 
energy  is  transformed  in  the  course  of  its  evolution 
and  appears  in  the  form  of  active  radiation. 

But  for  this  explanation  to  be  admitted,  it  would 
evidently  need  to  be  supported  by  very  numerous 
facts.  It  might,  no  doubt,  appear  still  more  prob- 
able that  the  energy  borrowed  from  the  external 
medium  by  radium  is  one  of  those  still  unknown  to 
us,  but  of  which  a  vague  instinct  causes  us  to  suspect 
the  existence  around  us.  It  is  indisputable,  more- 
over, that  the  atmosphere  in  all  directions  is 
furrowed  with  active  radiations;  those  of  radium 

1  In  reality  M.  Sagnac  operated  in  the  converse  manner. 
He  took  two  equal  weights  of  a  salt  of  radium  and  a  salt 
of  barium,  which  he  made  oscillate  one  after  the  other  in 
a  torsion  balance.  Had  the  durations  of  oscillation  been 
different,  it  might  be  concluded  that  the  mechanical  mass  is 
not  the  same  for  radium  as  for  barium, 


292    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

may  be  secondary  radiations  reflected  by  a  kind  of 
resonance  phenomenon. 

Certain  experiments  by  Professors  Elster  and 
Geitel,  however,  are  not  favourable  to  this  point  of 
view.  If  an  active  body  be  surrounded  by  a  radio- 
active envelope,  a  screen  should  prevent  this  body  from 
receiving  any  impression  from  outside,  and  yet  there 
is  no  diminution  apparent  in  the  activity  presented 
by  a  certain  quantity  of  radium  when  it  is  lowered 
to  a  depth  of  800  metres  under  ground,  in  a  region 
containing  a  notable  quantity  of  pitchblende.  These 
negative  results  are,  on  the  other  hand,  so  many 
successes  for  the  partisans  of  the  explanation  of 
radioactivity  by  atomic  energy. 


CHAPTEE  X 
THE  ETHER  AND  MATTER 

§  1.  THE  RELATIONS  BETWEEN  THE  ETHER 
AND  MATTER 

FOR  some  time  past  it  has  been  the  more  or  less 
avowed  ambition  of  physicists  to  construct  with  the 
particles  of  ether  all  possible  forms  of  corporeal 
existence ;  but  our  knowledge  of  the  inmost  nature 
of  things  has  hitherto  seemed  too  limited  for  us 
to  attempt  such  an  enterprise  with  any  chance 
of  success.  The  electronic  hypothesis,  however, 
which  has  furnished  a  satisfactory  image  of  the 
most  curious  phenomena  produced  in  the  bosom  of 
matter,  has  also  led  to  a  more  complete  electro- 
magnetic theory  of  the  ether  than  that  of  Maxwell, 
and  this  twofold  result  has  given  birth  to  the  hope 
of  arriving  by  means  of  this  hypothesis  at  a  complete 
co-ordination  of  the  physical  world. 

The  phenomena  whose  study  may  bring  us  to 
the  very  threshold  of  the  problem,  are  those  in 
which  the  connections  between  matter  and  the  ether 

293 


294    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

appear  clearly  and  in  a  relatively  simple  manner. 
Thus  in  the  phenomena  of  emission,  ponderable 
matter  is  seen  to  give  birth  to  waves  which  are 
transmitted  by  the  ether,  and  by  the  phenomena  of 
absorption  it  is  proved  that  these  waves  disappear 
and  excite  modifications  in  the  interior  of  the 
material  bodies  which  receive  them.  We  here  catch 
in  operation  actual  reciprocal  actions  and  reactions 
between  the  ether  and  matter.  If  we  could 
thoroughly  comprehend  these  actions,  we  should  no 
doubt  be  in  a  position  to  fill  up  the  gap  which 
separates  the  two  regions  separately  conquered  by 
physical  science. 

In  recent  years  numerous  researches  have  supplied 
valuable  materials  which  ought  to  be  utilized  by  those 
endeavouring  to  construct  a  theory  of  radiation. 
We  are,  perhaps,  still  ill  informed  as  to  the  pheno- 
mena of  luminescence  in  which  undulations  are 
produced  in  a  complex  manner,  as  in  the  case  of  a 
stick  of  moist  phosphorus  which  is  luminescent  in 
the  dark,  or  in  that  of  a  fluorescent  screen.  But  we 
are  very  well  acquainted  with  emission  or  absorption 
by  incandescence,  where  the  only  transformation  is 
that  of  calorific  into  radiating  energy,  or  vice  versa. 
It  is  in  this  case  alone  that  can  be  correctly  applied 
the  celebrated  demonstration  by  which  Kirchhoff 
established,  by  considerations  borrowed  from  thermo- 
dynamics, the  proportional  relations  between  the 
power  of  emission  and  that  of  absorption. 


THE  ETHER  AND  MATTER       295 

In  treating  of  the  measurement  of  temperature, 
I  have  already  pointed  out  the  experiments  of 
Professors  Lummer  and  Pringsheim  and  the  theoreti- 
cal researches  of  Stephan  and  Professor  Wien.  We 
may  consider  that  at  the  present  day  the  laws  of 
the  radiation  of  dark  bodies  are  tolerably  well 
known,  and,  in  particular,  the  manner  in  which 
each  elementary  radiation  increases  with  the 
temperature.  A  few  doubts,  however,  subsist  with 
respect  to  the  law  of  the  distribution  of  energy  in 
the  spectrum.  In  the  case  of  real  and  solid  bodies 
the  results  are  naturally  less  simple  than  in  that  of 
dark  bodies.  One  side  of  the  question  has  been 
specially  studied  on  account  of  its  great  practical 
interest,  that  is  to  say,  the  fact  that  the  relation  of 
the  luminous  energy  to  the  total  amount  radiated  by 
a  body  varies  with  the  nature  of  this  last ;  and  the 
knowledge  of  the  conditions  under  which  this  rela- 
tion becomes  most  considerable  led  to  the  discovery 
of  incandescent  lighting  by  gas  in  the  Auer-Welsbach 
mantle,  and  to  the  substitution  for  the  carbon  thread 
in  the  electric  light  bulb  of  a  filament  of  osmium 
or  a  small  rod  of  magnesium,  as  in  the  Nernst 
lamp.  Careful  measurements  effected  by  M.  Fery 
have  furnished,  in  particular,  important  informa- 
tion on  the  radiation  of  the  white  oxides ;  but  the 
phenomena  noticed  have  not  yet  found  a  satisfac- 
tory interpretation.  Moreover,  the  radiation  of 
calorific  origin  is  here  accompanied  by  a  more  or  less 


296    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

important  luminescence,  and  the  problem  becomes 
very  complex. 

In'  the  same  way  that,  for  the  purpose  of  knowing 
the  constitution  of  matter,  it  first  occurred  to  us 
to  investigate  gases,  which  appear  to  be  molecular 
edifices  built  on  a  more  simple  and  uniform  plan 
than  solids,  we  ought  naturally  to  think  that  an 
examination  of  the  conditions  in  which  emission  and 
absorption  are  produced  by  gaseous  bodies  might  be 
eminently  profitable,  and  might  perhaps  reveal  the 
mechanism  by  which  the  relations  between  the 
molecule  of  the  ether  and  the  molecule  of  matter 
might  be  established. 

Unfortunately,  if  a  gas  is  not  absolutely  incapable 
of  emitting  some  sort  of  rays  by  simple  heat,  the 
radiation  thus  produced,  no  doubt  by  reason  of  the 
slightness  of  the  mass  in  play,  always  remains  of 
moderate  intensity.  In  nearly  all  the  experiments, 
new  energies  of  chemical  or  electrical  origin  come 
into  force.  On  incandescence,  luminescence  is  super- 
posed ;  and  the  advantage  which  might  have 
been  expected  from  the  simplicity  of  the  medium 
vanishes  through  the  complication  of  the  circum- 
stances in  which  the  phenomenon  is  produced. 

Professor  Pringsheim  has  succeeded,  in  certain 
cases,  in  finding  the  dividing  line  between  the 
phenomena  of  luminescence  and  that  of  incan- 
descence. Thus  the  former  takes  a  predominating 
importance  when  the  gas  is  rendered  luminous  by 


THE  ETHER  AND  MATTER       297 

electrical  discharges,  and  chemical  transformations, 
especially,  play  a  preponderant  role  in  the  emission 
of  the  spectrum  of  flames  which  contain  a  saline 
vapour.  In  all  the  ordinary  experiments  of 
spectrum  analysis  the  laws  of  Kirchhoff  cannot 
therefore  be  considered  as  established,  and  yet  the 
relation  between  emission  and  absorption  is  generally 
tolerably  well  verified.  No  doubt  we  are  here  in 
presence  of  a  kind  of  resonance  phenomenon,  the 
gaseous  atoms  entering  into  vibration  when  solicited 
by  the  ether  by  a  motion  identical  with  the  one 
they  are  capable  of  communicating  to  it. 

If  we  are  not  yet  very  far  advanced  in  the 
study  of  the  mechanism  of  the  production  of  the 
spectrum,1  we  are,  on  the  other,  hand,  well  acquainted 
with  its  constitution.  The  extreme  confusion  which 
the  spectra  of  the  lines  of  the  gases  seemed  to 
present  is  now,  in  great  part  at  least,  cleared  up. 
Balmer  gave  some  time  since,  in  the  case  of  the 
hydrogen  spectrum,  an  empirical  formula  which 
enabled  the  rays  discovered  later  by  an  eminent 
astronomer,  M.  Deslandres,  to  be  represented ;  but 

1  Many  theories  as  to  the  cause  of  the  lines  and  bands  of  the 
spectrum  have  been  put  forward  since  this  was  written,  among 
which  that  of  Professor  Stark  (for  which  see  Physikalische 
Zeitschrift  for  1906,  passim)  is  perhaps  the  most  advanced. 
That  of  M.  Jean  Becquerel,  which  would  attribute  it  to  the 
vibration  within  the  atom  of  both  negative  and  positive 
electrons,  also  deserves  notice.  A  popular  account  of  this  is 
given  in  the  Athenxum  of  20th  April  1907. — ED. 


298    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

since  then,  both  in  the  cases  of  line  and  band  spectra, 
the  labours  of  Professor  Eydberg,  of  M.  Deslandres, 
of  Professors  Kayzer  and  Eunge,  and  of  M.  Thiele, 
have  enabled  us  to  comprehend,  in  their  smallest 
details,  the  laws  of  the  distribution  of  lines  and  bands. 

These  laws  are  simple,  but  somewhat  singular. 
The  radiations  emitted  by  a  gas  cannot  be  compared 
to  the  notes  to  which  a  sonorous  body  gives  birth, 
nor  even  to  the  most  complicated  vibrations  of  any 
elastic  body.  The  number  of  vibrations  of  the 
different  rays  are  not  the  successive  multiples  of 
one  and  the  same  number,  and  it  is  not  a  question 
of  a  fundamental  radiation  and  its  harmonics,  while 
— and  this  is  an  essential  difference — the  number 
of  vibrations  of  the  radiation  tend  towards  a  limit 
when  the  period  diminishes  infinitely  instead  of 
constantly  increasing,  as  would  be  the  case  with  the 
vibrations  of  sound. 

Thus  the  assimilation  of  the  luminous  to 
the  elastic  vibration  is  not  correct.  Once  again 
we  find  that  the  ether  does  not  behave  like 
matter  which  obeys  the  ordinary  laws  of  mechanics, 
and  every  theory  must  take  full  account  of  these 
curious  peculiarities  which  experiment  reveals. 

Another  difference,  likewise  very  important,  be- 
tween the  luminous  and  the  sonorous  vibrations, 
which  also  points  out  how  little  analogous  can  be 
the  constitutions  of  the  media  which  transmit  the 
vibrations,  appears  in  the  phenomena  of  dispersion. 


THE  ETHER  AND  MATTER       299 

The  speed  of  propagation,  which,  as  we  have  seen 
when  discussing  the  measurement  of  the  velocity  of 
sound,  depends  very  little  on  the  musical  note,  is  not 
at  all  the  same  in  the  case  of  the  various  radiations 
which  can  be  propagated  in  the  same  substance. 
The  index  of  refraction  varies  with  the  duration  of 
the  period,  or,  if  you  will,  with  the  length  of 
wave  in  vacuo  which  is  proportioned  to  this  dura- 
tion, since  in  vacuo  the  speed  of  propagation  is 
entirely  the  same  for  all  vibrations. 

Cauchy  was  the  first  to  propose  a  theory  on  which 
other  attempts  have  been  modelled ;  for  example, 
the  very  interesting  and  simple  one  of  Briot.  This 
last-named  supposed  that  the  luminous  vibration 
could  not  perceptibly  drag  with  it  the  molecular 
material  of  the  medium  across  which  it  is  propagated, 
but  that  matter,  nevertheless,  reacts  on  the  ether 
with  an  intensity  proportional  to  the  elongation,  in 
such  a  manner  as  tends  to  bring  it  back  to  its 
position  of  equilibrium.  With  this  simple  hypo- 
thesis we  can  fairly  well  interpret  the  phenomena 
of  the  dispersion  of  light  in  the  case  of  transparent 
substances ;  but  far  from  well,  as  M.  Carvallo  has 
noted  in  some  extremely  careful  experiments,  the 
dispersion  of  the  infra-red  spectrum,  and  not 
at  all  the  peculiarities  presented  by  absorbent 
substances. 

M.  Boussinesq  arrives  at  almost  similar  results, 
by  attributing  dispersion,  on  the  other  hand,  to  the 


300    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

partial  dragging  along  of  ponderable  matter  and  to 
its  action  on  the  ether.  By  combining,  in  a  measure, 
as  was  subsequently  done  by  M.  Boussinesq,  the  two 
hypotheses,  formulas  can  be  established  far  better  in 
accord  with  all  the  known  facts. 

These  facts  are  somewhat  complex.  It  was  at  first 
thought  that  the  index  always  varied  in  inverse 
ratio  to  the  wave-length,  but  numerous  substances 
have  been  discovered  which  present  the  phenomenon 
of  abnormal  dispersion — that  is  to  say,  substances 
in  which  certain  radiations  are  propagated,  on  the 
contrary,  the  more  quickly  the  shorter  their  period. 
This  is  the  case  with  gases  themselves,  as  demon- 
strated, for  example,  by  a  very  elegant  experiment 
of  M.  Becquerel  on  the  dispersion  of  the  vapour  of 
sodium.  Moreover,  it  may  happen  that  yet  more 
complications  may  be  met  with,  as  no  substance  is 
transparent  for  the  whole  extent  of  the  spectrum. 
In  the  case  of  certain  radiations  the  speed  of 
propagation  becomes  nil,  and  the  index  shows  some- 
times a  maximum  and  sometimes  a  minimum.  All 
those  phenomena  are  in  close  relation  with  those  of 
absorption. 

It  is,  perhaps,  the  formula  proposed  by  Helmholtz 
which  best  accounts  for  all  these  peculiarities. 
Helmholtz  came  to  establish  this  formula  by 
supposing  that  there  is  a  kind  of  friction  between 
the  ether  and  matter,  which,  like  that  exercised  on 
a  pendulum,  here  produces  a  double  effect,  changing, 


THE  ETHER  AND  MATTER  301 

on  the  one  hand,  the  duration  of  this  oscillation,  and, 
on  the  other,  gradually  damping  it.  He  further 
supposed  that  ponderable  matter  is  acted  on  by 
elastic  forces.  The  theory  of  Helmholtz  has  the 
great  advantage  of  representing,  not  only  the 
phenomena  of  dispersion,  but  also,  as  M.  Carvallo 
has  pointed  out,  the  laws  of  rotatory  polarization, 
its  dispersion  and  other  phenomena,  among  them 
the  dichroism  of  the  rotatory  media  discovered  by 
M.  Cotton. 

In  the  establishment  of  these  theories,  the 
language  of  ordinary  optics  has  always  been  em- 
ployed. The  phenomena  are  looked  upon  as  due  to 
mechanical  deformations  or  to  movements  governed 
by  certain  forces.  The  electromagnetic  theory  leads, 
as  we  have  seen,  to  the  employment  of  other 
images.  M.  H.  Poincare,  and,  after  him,  Helmholtz, 
have  both  proposed  electromagnetic  theories  of 
dispersion.  On  examining  things  closely,  it  will 
be  found  that  there  are  not,  in  truth,  in  the 
two  ways  of  regarding  the  problem,  two  equivalent 
translations  of  exterior  reality.  The  electrical 
theory  gives  us  to  understand,  much  better  than  the 
mechanical  one,  that  in  vacua  the  dispersion  ought 
to  be  strictly  null,  and  this  absence  of  dispersion 
appears  to  be  confirmed  with  extraordinary  precision 
by  astronomical  observations.  Thus  the  observation, 
often  repeated,  and  at  different  times  of  year,  proves 
that  in  the  case  of  the  star  Algol,  the  light  of 


302    THE   NEW  PHYSICS  AND  ITS  EVOLUTION 

which  takes  at  least  four  years  to  reach  us,  no 
sensible  difference  in  coloration  accompanies  the 
changes  in  brilliancy. 

§  2.  THE  THEORY  OF  LORENTZ 

Purely  mechanical  considerations  have  therefore 
failed  to  give  an  entirely  satisfactory  interpretation 
of  the  phenomena  in  which  even  the  simplest 
relations  between  matter  and  the  ether  appear. 
They  would,  evidently,  be  still  more  insufficient  if 
used  to  explain  certain  effects  produced  on  matter 
by  light,  which  could  not,  without  grave  difficulties, 
be  attributed  to  movement ;  for  instance,  the 
phenomena  of  electrification  under  the  influence 
of  certain  radiations,  or,  again,  chemical  reactions 
such  as  photographic  impressions. 

The  problem  had  to  be  approached  by  another 
road.  The  electromagnetic  theory  was  a  step  in 
advance,  but  it  comes  to  a  standstill,  so  to  speak,  at 
the  moment  when  the  ether  penetrates  into  matter. 
If  we  wish  to  go  deeper  into  the  inwardness  of  the 
phenomena,  we  must  follow,  for  example,  Professor 
Lorentz  or  Dr  Larmor,  and  look  with  them  for  a 
mode  of  representation  which  appears,  besides,  to  be 
a  natural  consequence  of  the  fundamental  ideas 
forming  the  basis  of  Hertz's  experiments. 

The  moment  we  look  upon  a  wave  in  the  ether  as 
an  electromagnetic  wave,  a  molecule  which  emits 
light  ought  to  be  considered  as  a  kind  of  excitant. 


THE  ETHER  AND  MATTER       303 

We  are  thus  led  to  suppose  that  in  each  radiating 
molecule  there  are  one  or  several  electrified 
particles,  animated  with  a  to -and -fro  movement 
round  their  positions  of  equilibrium,  and  these 
particles  are  certainly  identical  with  those  electrons 
the  existence  of  which  we  have  already  admitted  for 
so  many  other  reasons. 

In  the  simplest  theory,  we  will  imagine  an  elec- 
tron which  may  be  displaced  from  its  position  of 
equilibrium  in  all  directions,  and  is,  in  this  dis- 
placement, submitted  to  attractions  which  communi- 
cate to  it  a  vibration  like  a  pendulum.  These 
movements  are  equivalent  to  tiny  currents,  and  the 
mobile  electron,  when  animated  with  a  considerable 
velocity,  must  be  sensitive  to  the  action  of  the 
magnet  which  modifies  the  form  of  the  trajectory 
and  the  value  of  the  period.  This  almost  direct 
consequence  was  perceived  by  Lorentz,  and  it  led 
him  to  the  new  idea  that  radiations  emitted  by  a 
body  ought  to  be  modified  by  the  action  of  a  strong 
electromagnet. 

An  experiment  enabled  this  prevision  to  be 
verified.  It  was  made,  as  is  well  known,  as  early 
as  1896  by  Zeeman  ;  and  the  discovery  produced  a 
legitimate  sensation.  When  a  flame  is  subjected 
to  the  action  of  a  magnetic  field,  a  brilliant  line 
is  decomposed  in  conditions  more  or  less  complex 
which  an  attentive  study,  however,  allows  us  to 
define.  According  to  whether  the  observation  is 


304    THE   NEW  PHYSICS  AND   ITS  EVOLUTION 

made  in  a  plane  normal  to  the  magnetic  field  or 
in  the  same  direction,  the  line  transforms  itself  into 
a  triplet  or  doublet,  and  the  new  lines  are  polarized 
rectilinearly  or  circularly. 

These  are  the  precise  phenomena  which  the 
calculation  foretells :  the  analysis  of  the  modifica- 
tions undergone  by  the  light  supplies,  moreover, 
valuable  information  on  the  electron  itself.  From 
the  direction  of  the  circular  vibrations  of  the 
greatest  frequency  we  can  determine  the  sign  of  the 
electric  charge  in  motion  and  we  find  it  to  be 
negative.  But,  further  than  this,  from  the  varia- 
tion of  the  period  we  can  calculate  the  relation 
of  the  force  acting  on  the 'electron  to  its  material 
mass,  and,  in  addition,  the  relation  of  the  charge  to 
the  mass.  We  then  find  for  this  relation  precisely 
that  value  which  we  have  already  met  with  so  many 
times.  Such  a  coincidence  cannot  be  fortuitous, 
and  we  have  the  right  to  believe  that  the  electron 
revealed  by  the  luminous  wave  which  emanates 
from  it,  is  really  the  same  as  the  one  made  known 
to  us  by  the  study  of  the  cathode  rays  and  of  the 
radioactive  substances. 

However,  the  elementary  theory  does  not  suffice 
to  interpret  the  complications  which  later  experi- 
ments have  revealed.  The  physicists  most  qualified 
to  effect  measurements  in  these  delicate  optical 
questions — M.  Cornu,  Mr  Preston,  M.  Cotton,  MM. 
Becquerel  and  JDeslandres,  M.  Broca,  Professor 


THE  ETHEE  AND  MATTER       305 

Michelson,  and  others — have  pointed  out  some  re- 
markable peculiarities.  Thus  in  some  cases  the 
number  of  the  component  rays  dissociated  by  the 
magnetic  field  may  be  very  considerable. 

The  great  modification  brought  to  a  radiation  by 
the  Zeeman  effect  may,  besides,  combine  itself  with 
other  phenomena,  and  alter  the  light  in  a  still  more 
complicated  manner.  A  pencil  of  polarized  light, 
as  demonstrated  by  Signori  Macaluzo  and  Corbino, 
undergoes,  in  a  magnetic  field,  modifications  with 
regard  to  absorption  and  speed  of  propagation. 

Some  ingenious  researches  by  M.  Becquerel  and  M. 
Cotton  have  perfectly  elucidated  all  these  complica- 
tions from  an  experimental  point  of  view.  It  would 
not  be  impossible  to  link  together  all  these  pheno- 
mena without  adopting  the  electronic  hypothesis,  by 
preserving  the  old  optical  equations  as  modified  by 
the  terms  relating  to  the  action  of  the  magnetic  field. 
This  has  actually  been  done  in  some  very  remarkable 
work  by  M.  Voigt,  but  we  may  also,  like  Professor 
Lorentz,  look  for  more  general  theories,  in  which  the 
essential  image  of  the  electrons  shall  be  preserved, 
and  which  will  allow  all  the  facts  revealed  by  ex- 
periment to  be  included. 

We  are  thus  led  to  the  supposition  that  there  is 
not  in  the  atom  one  vibrating  electron  only,  but 
that  there  is  to  be  found  in  it  a  dynamical  system 
comprising  several  material  points  which  may  be 

subjected  to  varied  movements.     The  neutral  atom 

20 


306    THE  NEW  PHYSICS  AND  ITS   EVOLUTION 

may  therefore  be  considered  as  composed  of  an  im- 
mctvable  principal  portion  positively  charged,  round 
which  move,  like  satellites  round  a  planet,  several 
negative  electrons  of  very  inferior  mass.  This  con- 
clusion leads  us  to  an  interpretation  in  agreement 
with  that  which  other  phenomena  have  already 
suggested. 

These  electrons,  which  thus  have  a  variable  velocity, 
generate  around  themselves  a  transverse  electro- 
magnetic wave  which  is  propagated  with  the  velocity 
of  light ;  for  the  charged  particle  becomes,  as  soon 
as  it  experiences  a  change  of  speed,  the  centre  of  a 
radiation.  Thus  is  explained  the  phenomenon  of  the 
emission  of  radiations.  In  the  same  way,  the  move- 
ment of  electrons  may  be  excited  or  modified  by  the 
electrical  forces  which  exist  in  any  pencil  of  light  they 
receive,  and  this  pencil  may  yield  up  to  them  a  part 
of  the  energy  it  is  carrying.  This  is  the  phenomenon 
of  absorption. 

Professor  Lorentz  has  not  contented  himself  with 
thus  explaining  all  the  mechanism  of  the  phenomena 
of  emission  and  •  absorption.  He  has  endeavoured 
to  rediscover,  by  starting  with  the  fundamental 
hypothesis,  the  quantitative  laws  discovered  by 
thermodynamics.  He  succeeds  in  showing  that, 
agreeably  to  the  law  of  Kirchhoff,  the  relation 
between  the  emitting  and  the  absorbing  power  must 
be  independent  of  the  special  properties  of  the 
body  under  observation,  and  he  thus  again  meets 


THE  ETHER  AND  MATTER       307 

with  the  laws  of  Planck  and  of  Wien :  unfortunately 
the  calculation  can  only  be  made  in  the  case  of  great 
wave-lengths,  and  grave  difficulties  exist.  Thus  it 
cannot  be  very  clearly  explained  why,  by  heating  a 
body,  the  radiation  is  displaced  towards  the  side  of 
the  short  wave-lengths,  or,  if  you  will,  why  a  body 
becomes  luminous  from  the  moment  its  temperature 
has  reached  a  sufficiently  high  degree.  On  the  other 
hand,  by  calculating  the  energy  of  the  vibrating 
particles  we  are  again  led  to  attribute  to  these 
particles  the  same  constitution  as  that  of  the 
electrons. 

It  is  in  the  same  way  possible,  as  Professor  Lorentz 
has  shown,  to  give  a  very  satisfactory  explanation  of 
the  thermo-electric  phenomena  by  supposing  that 
the  number  of  liberated  electrons  which  exist  in 
a  given  metal  at  a  given  temperature  has  a  deter- 
mined value  varying  with  each  metal,  and  is,  in 
the  case  of  each  body,  a  function  of  the  temperature. 
The  formula  obtained,  which  is  based  on  these  hypo- 
theses, agrees  completely  with  the  classic  results  of 
Clausius  and  of  Lord  Kelvin.  Finally,  if  we  recollect 
that  the  phenomena  of  electric  and  calorific  conduc- 
tivity are  perfectly  interpreted  by  the  hypothesis 
of  electrons,  it  will  no  longer  be  possible  to  contest 
the  importance  of  a  theory  which  allows  us  to  group 
together  in  one  synthesis  so  many  facts  of  such 
diverse  origins. 

If  we  study  the  conditions  under  which  a  wave 


3o8    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

excited  by  an  electron's  variations  in  speed  can  be  trans- 
mitted, they  again  bring  us  face  to  face,  and  gener- 
ally, with  the  results  pointed  out  by  the  ordinary 
electromagnetic  theory.  Certain  peculiarities,  how- 
ever, are  not  absolutely  the  same.  Thus  the  theory 
of  Lorentz,  as  well  as  that  of  Maxwell,  leads  us  to 
foresee  that  if  an  insulating  mass  be  caused  to  move 
in  a  magnetic  field  normally  to  its  lines  of  force,  a 
displacement  will  be  produced  in  this  mass  analogous 
to  that  of  which  Faraday  and  Maxwell  admitted  the 
existence  in  the  dielectric  of  a  charged  condenser. 
But  M.  H.  Poincare  has  pointed  out  that,  according 
as  we  adopt  one  or  other  of  these  authors'  points  of 
view,  so  the  value  of  the  displacement  differs.  This 
remark  is  very  important,  for  it  may  lead  to  an 
experiment  which  would  enable  us  to  make  a 
definite  choice  between  the  two  theories. 

To  obtain  the  displacement  estimated  according  to 
Lorentz,  we  must  multiply  the  displacement  calcu- 
lated according  to  Hertz  by  a  factor  representing 
the  relation  between  the  difference  of  the  specific  in- 
ductive capacities  of  the  dielectric  and  of  a  vacuum, 
and  the  first  of  these  powers.  If  therefore  we  take 
as  dielectric  the  air  of  which  the  specific  induc- 
tive capacity  is  perceptibly  the  same  as  that  of  a 
vacuum,  the  displacement,  according  to  the  idea 
of  Lorentz,  will  be  null ;  while,  on  the  contrary, 
according  to  Hertz,  it  will  have  a  finite  value.  M. 
Blondlot  has  made  the  experiment.  He  sent  a 


THE  ETHER  AND  MATTER       309 

current  of  air  into  a  condenser  placed  in  a  magnetic 
field,  and  was  never  able  to  notice  the  slightest 
trace  of  electrification.  No  displacement,  therefore, 
is  effected  in  the  dielectric.  The  experiment  being 
a  negative  one,  is  evidently  less  convincing  than  one 
giving  a  positive  result,  but  it  furnishes  a  very 
powerful  argument  in  favour  of  the  theory  of 
Lorentz. 

This  theory,  therefore,  appears  very  seductive,  yet 
it  still  raises  objections  on  the  part  of  those  who 
oppose  to  it  the  principles  of  ordinary  mechanics. 
If  we  consider,  for  instance,  a  radiation  emitted  by  an 
electron  belonging  to  one  material  body,  but  absorbed 
by  another  electron  in  another  body,  we  perceive 
immediately  that,  the  propagation  not  being  instan- 
taneous, there  can  be  no  compensation  between  the 
action  and  the  reaction,  which  are  not  simultaneous ; 
and  the  principle  of  Newton  thus  seems  to  be 
attacked.  In  order  to  preserve  its  integrity,  it  has  to 
be  admitted  that  the  movements  in  the  two  material 
substances  are  compensated  by  that  of  the  ether 
which  separates  these  substances ;  but  this  concep- 
tion, although  in  tolerable  agreement  with  the 
hypothesis  that  the  ether  and  matter  are  not  of 
different  essence,  involves,  on  a  closer  examination, 
suppositions  hardly  satisfactory  as  to  the  nature  of 
movements  in  the  ether. 

For  a  long  time  physicists  have  admitted  that 
the  ether  as  a  whole  must  be  considered  as  being 


310    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

immovable  and  capable  of  serving,  so  to  speak,  as  a 
support  for  the  axes  of  Galileo,  in  relation  to  which 
axes  the  principle  of  inertia  is  applicable, — or  better 
still,  as  M.  Painleve'  has  shown,  they  alone  allow  us 
to  render  obedience  to  the  principle  of  causality. 

But  if  it  were  so,  we  might  apparently  hope,  by 
experiments  in  electromagnetism,  to  obtain  absolute 
motion,  and  to  place  in  evidence  the  translation 
of  the  earth  relatively  to  the  ether.  But  all  the 
researches  attempted  by  the  most  ingenious  physicists 
towards  this  end  have  always  failed,  and  this  tends 
towards  the  idea  held  by  many  geometricians  that 
these  negative  results  are  not  due  to  imperfections  in 
the  experiments,  but  have  a  deep  and  general  cause. 
Now  Lorentz  has  endeavoured  to  find  the  conditions 
in  which  the  electromagnetic  theory  proposed  by 
him  might  agree  with  the  postulate  of  the  complete 
impossibility  of  determining  absolute  motion.  It  is 
necessary,  in  order  to  realise  this  concord,  to  imagine 
that  a  mobile  system  contracts  very  slightly  in  the 
direction  of  its  translation  to  a  degree  proportioned 
to  the  square  of  the  ratio  of  the  velocity  of  transport 
to  that  of  light.  The  electrons  themselves  do  not 
escape  this  contraction,  although  the  observer,  since 
he  participates  in  the  same  motion,  naturally  cannot 
notice  it.  Lorentz  supposes,  besides,  that  all  forces, 
whatever  their  origin,  are  affected  by  a  translation  in 
the  same  way  as  electromagnetic  forces.  M.  Langevin 
and  M.  H.  Poincar^  have  studied  this  same  question 


THE  ETHER  AND  MATTER       311 

and  have  noted  with  precision  various  delicate  conse- 
quences of  it.  The  singularity  of  the  hypotheses  which 
we  are  thus  led  to  construct  in  no  way  constitutes  an 
argument  against  the  theory  of  Lorentz  ;  but  it  has, 
we  must  acknowledge,  discouraged  some  of  the  more 
timid  partisans  of  this  theory.1 

§  3.  THE  MASS  OF  ELECTRONS 

Other  conceptions,  bolder  still,  are  suggested  by 
the  results  of  certain  interesting  experiments.  The 
electron  affords  us  the  possibility  of  considering 
inertia  and  mass  to  be  no  longer  a  fundamental 
notion,  but  a  consequence  of  the  electromagnetic 
phenomena. 

Professor  J.  J.  Thomson  was  the  first  to  have 
the  clear  idea  that  a  part,  at  least,  of  the  inertia  of 
an  electrified  body  is  due  to  its  electric  charge.  This 
idea  was  taken  up  and  precisely  stated  by  Professor 
Max  Abraham,  who,  for  the  first  time,  was  led  to 

1  An  objection  not  here  noticed  has  lately  been  formulated 
with  much  frankness  by  Professor  Lorentz  himself.  It  is  one 
of  the  pillars  of  his  theory  that  only  the  negative  electrons 
move  when  an  electric  current  passes  through  a  metal,  and 
that  the  positive  electrons  (if  any  such  there  be)  remain 
motionless.  Yet  in  the  experiment  known  as  Hall's,  the 
current  is  deflected  by  the  magnetic  field  to  one  side  of  the 
strip  in  certain  metals,  and  to  the  opposite  side  in  others. 
This  seems  to  show  that  in  certain  cases  the  positive  electrons 
move  instead  of  the  negative,  and  Professor  Lorentz  confesses 
that  up  to  the  present  he  can  find  no  valid  argument  against 
this.  See  Archives  Ne'erlandaises  1906,  parts  1  and  2. — ED. 


3i2    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

regard  seriously  the  seemingly  paradoxical  notion 
of  mass  as  a  function  of  velocity.  Consider  a  small 
particle  bearing  a  given  electric  charge,  and  let  us 
suppose  that  this  particle  moves  through  the  ether. 
It  is,  as  we  know,  equivalent  to  a  current  proportional 
to  its  velocity,  and  it  therefore  creates  a  magnetic 
field  the  intensity  of  which  is  likewise  proportional 
to  its  velocity:  to  set  it  in  motion,  therefore,  there 
must  be  communicated  to  it  over  and  above  the 
expenditure  corresponding  to  the  acquisition  of  its 
ordinary  kinetic  energy,  a  quantity  of  energy  pro- 
portional to  the  square  of  its  velocity.  Everything, 
therefore,  takes  place  as  if,  by  the  fact  of  electrifica- 
tion, its  capacity  for  kinetic  energy  and  its  material 
mass  had  been  increased  by  a  certain  constant  quan- 
tity. To  the  ordinary  mass  may  be  added,  if  you  will, 
an  electromagnetic  mass. 

This  is  the  state  of  things  so  long  as  the  speed  of 
the  translation  of  the  particle  is  not  very  great,  but 
they  are  no  longer  quite  the  same  when  this  particle 
is  animated  with  a  movement  whose  rapidity  becomes 
comparable  to  that  with  which  light  is  propagated. 

The  magnetic  field  created  is  then  no  longer  a 
field  in  repose,  but  its  energy  depends,  in  a  compli- 
cated manner,  on  the  velocity,  and  the  apparent 
increase  in  the  mass  of  the  particle  itself  becomes 
a  function  of  the  velocity.  More  than  this,  this 
increase  may  not  be  the  same  for  the  same  velocity, 
but  varies  according  to  whether  the  acceleration  is 


THE  ETHER  AND  MATTER       313 

parallel  with  or  perpendicular  to  the  direction  of  this 
velocity.  In  other  words,  there  seems  to  be  a  longi- 
tudinal- and  a  transversal  mass  which  need  not  be 
the  same. 

All  these  results  would  persist  even  if  the  material 
mass  were  very  small  relatively  to  the  electro- 
magnetic mass ;  and  the  electron  possesses  some  in- 
ertia even  if  its  ordinary  mass  becomes  slighter  and 
slighter.  The  apparent  mass,  it  can  be  easily  shown, 
increases  indefinitely  when  the  velocity  with  which 
the  electrified  particle  is  animated  tends  towards 
the  velocity  of  light,  and  thus  the  work  necessary 
to  communicate  such  a  velocity  to  an  electron  would 
be  infinite.  It  is  in  consequence  impossible  that  the 
speed  of  an  electron,  in  relation  to  the  ether,  can 
ever  exceed,  or  even  permanently  attain  to,  300,000 
kilometres  per  second. 

All  the  facts  thus  predicted  by  the  theory  are 
confirmed  by  experiment.  There  is  no  known 
process  which  permits  the  direct  measurement  of 
the  mass  of  an  electron,  but  it  is  possible,  as  we  have 
seen,  to  measure  simultaneously  its  velocity  and 
the  relation  of  the  electric  charge  to  its  mass.  In 
the  case  of  the  cathode  rays  emitted  by  radium, 
these  measurements  are  particularly  interesting,  for 
the  reason  that  the  rays  which  compose  a  pencil 
of  cathode  rays  are  animated  by  very  different  speeds, 
as  is  shown  by  the  size  of  the  stain  produced  on  a 
photographic  plate  by  a  pencil  of  them  at  first  very 


3i4    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

constricted  and  subsequently  dispersed  by  the  action 
of  an  electric  or  magnetic  field.  Professor  Kauf  mann 
has  effected  some  very  careful  experiments  by  a 
method  he  terms  the  method  of  crossed  spectra, 
which  consists  in  superposing  the  deviations  pro- 
duced by  a  magnetic  and  an  electric  field  respectively 
acting  in  directions  at  right  angles  one  to  another. 
He  has  thus  been  enabled  by  working  in  vaciw  to 
register  the  very  different  velocities  which,  starting 
in  the  case  of  certain  rays  from  about  seven-tenths 
of  the  velocity  of  light,  attain  in  other  cases  to 
ninety-five  hundredths  of  it. 

It  is  thus  noted  that  the  ratio  of  charge  to  mass 
— which  for  ordinary  speeds  is  constant  and  equal 
to  that  already  found  by  so  many  experiments — 
diminishes  slowly  at  first,  and  then  very  rapidly  when 
the  velocity  of  the  ray  increases  and  approaches  that 
of  light.  If  we  represent  this  variation  by  a  curve, 
the  shape  of  this  curve  inclines  us  to  think  that  the 
ratio  tends  toward  zero  when  the  velocity  tends 
towards  that  of  light. 

All  the  earlier  experiments  have  led  us  to  con- 
sider that  the  electric  charge  was  the  same  for  all 
electrons,  and  it  can  hardly  be  conceived  that  this 
charge  can  vary  with  the  velocity.  For  in  order  that 
the  relation,  of  which  one  of  the  terms  remains  fixed, 
should  vary,  the  other  term  necessarily  cannot  remain 
constant.  The  experiments  of  Professor  Kaufmann, 
therefore,  confirm  the  previsions  of  Max  Abraham's 


THE  ETHER  AND  MATTER       315 

theory:  the  mass  depends  on  the  velocity,  and 
increases  indefinitely  in  proportion  as  this  velocity 
approaches  that  of  light.  These  experiments,  more- 
over, allow  the  numerical  results  of  the  calculation 
to  be  compared  with  the  values  measured.  This 
very  satisfactory  comparison  shows  that  the  apparent 
total  mass  is  sensibly  equal  to  the  electromagnetic 
mass  ;  the  material  mass  of  the  electron  is  therefore 
nil,  and  the  whole  of  its  mass  is  electromagnetic. 

Thus  the  electron  must  be  looked  upon  as  a  simple 
electric  charge  devoid  of  matter.  Previous  examina- 
tion has  led  us  to  attribute  to  it  a  mass  a  thousand 
times  less  that  that  of  the  atom  of  hydrogen,  and 
a  more  attentive  study  shows  that  this  mass  was 
fictitious.  The  electromagnetic  phenomena  which 
are  produced  when  the  electron  is  set  in  motion  or 
a  change  effected  in  its  velocity,  simply  have  the 
effect,  as  it  were,  of  simulating  inertia,  and  it  is 
the  inertia  due  to  the  charge  which  has  caused  us 
to  be  thus  deluded. 

The  electron  is  therefore  simply  a  small  volume 
determined  at  a  point  in  the  ether,  and  possessing 
special  properties ; l  this  point  is  propagated  with  a 
velocity  which  cannot  exceed  that  of  light.  When  this 
velocity  is  constant,  the  electron  creates  around  it 
in  its  passage  an  electric  and  a  magnetic  field  ;  round 
this  electrified  centre  there  exists  a  kind  of  wake, 

1  This  cannot  be  said  to  be  yet  completely  proved.     Of.  Sir 
Oliver  Lodge,  Electrons,  London,  1906,  p.  200.— ED. 


316    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

which  follows  it  through  the  ether  and  does  not 
become  modified  so  long  as  the  velocity  remains 
invariable.  If  other  electrons  follow  the  first  within 
a  wire,  their  passage  along  the  wire  will  be  what  is 
called  an  electric  current. 

When  the  electron  is  subjected  to  an  acceleration, 
a  transverse  wave  is  produced,  and  an  electro- 
magnetic radiation  is  generated,  of  which  the 
character  may  naturally  change  with  the  manner 
in  which  the  speed  varies.  If  the  electron  has  a 
sufficiently  rapid  periodical  movement,  this  wave  is 
a  light  wave  ;  while  if  the  electron  stops  suddenly,  a 
kind  of  pulsation  is  transmitted  through  the  ether, 
and  thus  we  obtain  Eontgen  rays. 

§  4.  NEW  VIEWS  ON  THE  CONSTITUTION  OF  THE 
ETHER  AND  OF  MATTER 

New  and  valuable  information  is  thus  afforded  us 
regarding  the  properties  of  the  ether,  but  will  this 
enable  us  to  construct  a  material  representation  of 
this  medium  which  fills  the  universe,  and  so  to  solve 
a  problem  which  has  baffled,  as  we  have  seen,  the 
prolonged  efforts  of  our  predecessors  ? 

Certain  scholars  seem  to  have  cherished  this  hope. 
Dr  Larmor  in  particular,  as  we  have  seen,  has 
proposed  a  most  ingenious  image,  but  one  which  is 
manifestly  insufficient.  The  present  tendency  of 
physicists  rather  tends  to  the  opposite  view ;  since 
they  consider  matter  as  a  very  complex  object, 


THE   ETHER  AND   MATTER  317 

regarding  which  we  wrongly  imagine  ourselves  to  be 
well  informed  because  we  are  so  much  accustomed 
to  it,  and  its  singular  properties  end  by  seeming 
natural  to  us.  But  in  all  probability  the  ether  is, 
in  its  objective  reality,  much  more  simple,  and  has 
a  better  right  to  be  considered  as  fundamental. 

We  cannot  therefore,  without  being  very 
illogical,  define  the  ether  by  material  properties, 
and  it  is  useless  labour,  condemned  beforehand  to 
sterility,  to  endeavour  to  determine  it  by  other 
qualities  than  those  of  which  experiment  gives  us 
direct  and  exact  knowledge. 

The  ether  is  defined  when  we  know,  in  all  its 
points,  and  in  magnitude  and  in  direction,  the  two 
fields,  electric  and  magnetic,  which  may  exist  in  it. 
These  two  fields  may  vary ;  we  speak  from  habit 
of  a  movement  propagated  in  the  ether,  but  the 
phenomenon  within  the  reach  of  experiment  is  the 
propagation  of  these  variations. 

Since  the  electrons,  considered  as  a  modification 
of  the  ether  symmetrically  distributed  round  a 
point,  perfectly  counterfeit  that  inertia  which  is  the 
fundamental  property  of  matter,  it  becomes  very 
tempting  to  suppose  that  matter  itself  is  composed 
of  a  more  or  less  complex  assemblage  of  electrified 
centres  in  motion. 

This  complexity  is,  in  general,  very  great,  as  is 
demonstrated  by  the  examination  of  the  luminous 
spectra  produced  by  the  atoms,  and  it  is  precisely 


318    THE  NEW   PHYSICS  AND   ITS  EVOLUTION 

because  of  the  compensations  produced  between  the 
different  movements  that  the  essential  properties  of 
matter — the  law  of  the  conservation  of  inertia,  for 
example — are  not  contrary  to  the  hypothesis. 

The  forces  of  cohesion  thus  would  be  due  to  the 
mutual  attractions  which  occur  in  the  electric  and 
magnetic  fields  produced  in  the  interior  of  bodies;  and 
it  is  even  conceivable  that  there  may  be  produced, 
under  the  influence  of  these  actions,  a  tendency  to 
determine  orientation,  that  is  to  say,  that  a  reason 
can  be  seen  why  matter  may  be  crystallised.1 

All  the  experiments  effected  on  the  conductivity 
of  gases  or  metals,  and  on  the  radiations  of  active 
bodies,  have  induced  us  to  regard  the  atom  as  being 
constituted  by  a  positively  charged  centre  having 
practically  the  same  magnitude  as  the  atom  itself, 
round  which  the  electrons  gravitate  ;  and  it  might 
evidently  be  supposed  that  this  positive  centre  itself 
preserves  the  fundamental  characteristics  of  matter, 
and  that  it  is  the  electrons  alone  which  no  longer 
possess  any  but  electromagnetic  mass. 

We  have  but  little  information  concerning  these 
positive  particles,  though  they  are  met  with  in  an 
isolated  condition,  as  we  have  seen,  in  the  canal  rays 


1  The  reader  should,  however,  be  warned  that  a  theory  has 
lately  been  put  forth  which  attempts  to  account  for  crystallisa- 
tion on  purely  mechanical  grounds.  See  Messrs  Barlow  and 
Pope's  "Development  of  the  Atomic  Theory"  in  the  Trans- 
actions of  the  Chemical  Society,  1906.— ED. 


THE  ETHER  AND  MATTER       319 

or  in  the  X  rays.1  It  has  not  hitherto  been  possible 
to  study  them  so  successfully  as  the  electrons  them- 
selves ;  but  that  their  magnitude  causes  them  to 
produce  considerable  perturbations  in  the  bodies 
on  which  they  fall  is  manifest  by  the  secondary 
emissions  which  complicate  and  mask  the  primitive 
phenomenon.  There  are,  however,  strong  reasons  for 
thinking  that  these  positive  centres  are  not  simple. 
Thus  Professor  Stark  attributes  to  them,  with  experi- 
ments in  proof  of  his  opinion,  the  emission  of  the 
spectra  of  the  rays  in  Geissler  tubes,  and  the  com- 
plexity of  the  spectrum  discloses  the  complexity  of 
the  centre.  Besides,  certain  peculiarities  in  the 
conductivity  of  metals  cannot  be  explained  without 
a  supposition  of  this  kind.  So  that  the  atom,  de- 
prived of  the  cathode  corpuscle,  would  be  still  liable 
to  decomposition  into  elements  analogous  to  electrons 
and  positively  charged.  Consequently  nothing  pre- 
vents us  supposing  that  this  centre  likewise  simulates 
inertia  by  its  electromagnetic  properties,  and  is  but 
a  condition  localised  in  the  ether. 

However  this  may  be,  the  edifice  thus  constructed, 
being  composed  of  electrons  in  periodical  motion, 
necessarily  grows  old.  The  electrons  become  subject 

1  There  is  much  reason  for  thinking  that  the  canal  rays  do 
not  contain  positive  particles  alone,  but  are  accompanied  by 
negative  electrons  of  slow  velocity.  The  X  rays  are  thought, 
as  has  been  said  above,  to  contain  neither  negative  nor  positive 
particles,  but  to  be  merely  pulses  in  the  ether. — ED. 


320    THE  NEW  PHYSICS  AND   ITS  EVOLUTION 

to  accelerations  which  produce  a  radiation  towards 
the  exterior  of  the  atom ;  and  certain  of  them  may 
leave  the  body,  while  the  primitive  stability  is,  in 
the  end,  no  longer  assured,  and  a  new  arrangement 
tends  to  be  formed.  Matter  thus  seems  to  us  to 
undergo  those  transformations  of  which  the  radio-active 
bodies  have  given  us  such  remarkable  examples. 

We  have  already  had,  in  fragments,  these  views 
on  the  constitution  of  matter  ;  a  deeper  study  of  the 
electron  thus  enables  us  to  take  up  a  position  from 
which  we  obtain  a  sharp,  clear,  and  comprehensive 
grasp  of  the  whole  and  a  glimpse  of  indefinite 
horizons. 

It  would  be  advantageous,  however,  in  order  to 
strengthen  this  position,  that  a  few  objections  which 
still  menace  it  should  be  removed.  The  instability 
of  the  electron  is  not  yet  sufficiently  demonstrated. 
How  is  it  that  its  charge  does  not  waste  itself 
away,  and  what  bonds  assure  the  permanence  of  its 
constitution  ? 

On  the  other  hand,  the  phenomena  of  gravita- 
tion remain  a  mystery.  Lorentz  has  endeavoured  to 
build  up  a  theory  in  which  he  explains  attraction 
by  supposing  that  two  charges  of  similar  sign  repel 
each  other  in  a  slightly  less  degree  than  that  in 
which  two  charges,  equal  but  of  contrary  sign,  attract 
each  other,  the  difference  being,  however,  according 
to  the  calculation,  much  too  small  to  be  directly 
observed.  He  has  also  sought  to  explain  gravitation 


THE  ETHER  AND  MATTER       321 

by  connecting  it  with  the  pressures  which  may 
be  produced  on  bodies  by  the  vibratory  move- 
ments which  form  very  penetrating  rays.  Eecently 
M.  Sutherland  has  imagined  that  attraction  is 
due  to  the  difference  of  action  in  the  convection 
currents  produced  by  the  positive  and  negative  cor- 
puscles which  constitute  the  atoms  of  the  stars,  and 
are  carried  along  by  the  astronomical  motions.  But 
these  hypotheses  remain  rather  vague,  and  many 
authors  think,  like  M.  Langevin,  that  gravitation 
must  result  from  some  mode  of  activity  of  the  ether 
totally  different  from  the  electromagnetic  mode. 


21 


CHAPTEE  XI 
THE  FUTURE  OF  PHYSICS 

IT  would  doubtless  be  exceedingly  rasb,  and 
certainly  very  presumptuous,  to  seek  to  predict  the 
future  which  may  be  reserved  for  physics.  The 
role  of  prophet  is  not  a  scientific  one,  and  the 
most  firmly  established  previsions  of  to-day  may  be 
overthrown  by  the  reality  of  to-morrow. 

Nevertheless,  the  physicist  does  not  shun  an 
extrapolation  of  some  little  scope  when  it  is  not  too 
far  from  the  realms  of  experiment ;  the  knowledge 
of  the  evolution  accomplished  of  late  years  authorises 
a  few  suppositions  as  to  the  direction  in  which 
progress  may  continue. 

The  reader  who  has  deigned  to  follow  me  in  the 
rapid  excursion  we  have  just  made  through  the 
domain  of  the  science  of  Nature,  will  doubtless  bring 
back  with  him  from  his  short  journey  the  general 
impression  that  the  ancient  limits  to  which  the 
classic  treatises  still  delight  in  restricting  the 
divers  chapters  of  physics,  are  trampled  down  in 

all  directions. 

322 


THE  FUTURE  OF  PHYSICS  323 

The  fine  straight  roads  traced  out  by  the  masters 
of  the  last  century,  and  enlarged  and  levelled  by  the 
labour  of  such  numbers  of  workmen,  are  now  joined 
together  by  a  crowd  of  small  paths  which  furrow  the 
field  of  physics.  It  is  not  only  because  they  cover 
regions  as  yet  little  explored  where  discoveries  are 
more  abundant  and  more  easy,  that  these  cross-cuts 
are  so  frequent,  but  also  because  a  higher  hope  guides 
the  seekers  who  engage  in  these  new  routes. 

In  spite  of  the  repeated  failures  which  have  followed 
the  numerous  attempts  of  past  times,  the  idea  has 
not  been  abandoned  of  one  day  conquering  the 
supreme  principle  which  must  command  the  whole 
of  physics. 

Some  physicists,  no  doubt,  think  such  a  synthesis 
to  be  impossible  of  realisation,  and  that  Nature  is 
infinitely  complex ;  but,  notwithstanding  all  the 
reserves  they  may  make,  from  the  philosophical  point 
of  view,  as  to  the  legitimacy  of  the  process,  they 
do  not  hesitate  to  construct  general  hypotheses 
which,  in  default  of  complete  mental  satisfaction,  at 
least  furnish  them  with  a  highly  convenient  means 
of  grouping  an  immense  number  of  facts  till  then 
scattered  abroad. 

Their  error,  if  error  there  be,  is  beneficial,  for  it 
is  one  of  those  that  Kant  would  have  classed  among 
the  fruitful  illusions  which  engender  the  indefinite 
progress  of  science  and  lead  to  great  and  important 
co-ordinations. 


324    THE   NEW  PHYSICS   AND  ITS  EVOLUTION 

It  is,  naturally,  by  the  study  of  the  relations  exist- 
ing between  phenomena  apparently  of  very  different 
orders  that  there  can  be  any  hope  of  reaching  the 
goal ;  and  it  is  this  which  justifies  the  peculiar 
interest  accorded  to  researches  effected  in  the 
debatable  land  between  domains  hitherto  considered 
as  separate. 

Among  all  the  theories  lately  proposed,  that  of  the 
ions  has  taken  a  preponderant  place  ;  ill  understood 
at  first  by  some,  appearing  somewhat  singular,  and  in 
any  case  useless,  to  others,  it  met  at  its  inception, 
in  France  at  least,  with  only  very  moderate  favour. 

To-day  things  have  greatly  changed,  and  those 
even  who  ignored  it  have  been  seduced  by  the 
curious  way  in  which  it  adapts  itself  to  the  inter- 
pretation of  the  most  recent  experiments  on  very 
different  subjects.  A  very  natural  reaction  has  set 
in;  and  I  might  almost  say  that  a  question  of 
fashion  has  led  to  some  exaggerations. 

The  electron  has  conquered  physics,  and  many 
adore  the  new  idol  rather  blindly.  Certainly  we 
can  only  bow  before  an  hypothesis  which  enables 
us  to  group  in  the  same  synthesis  all  the  dis- 
coveries on  electric  discharges  and  on  radio- 
active substances,  and  which  leads  to  a  satisfactory 
theory  of  optics  and  of  electricity  ;  while  by  the 
intermediary  of  radiating  heat  it  seems  likely  to 
embrace  shortly  the  principles  of  thermodynamics 
also.  Certainly  one  must  admire  the  power  of  a 


THE  FUTURE   OF  PHYSICS  325 

creed  which  penetrates  also  into  the  domain  of 
mechanics  and  furnishes  a  simple  representation  of 
the  essential  properties  of  matter;  but  it  is  right 
not  to  lose  sight  of  the  fact  that  an  image  may  be  a 
well-founded  appearance,  but  may  not  be  capable  of 
being  exactly  superposed  on  the  objective  reality. 

The  conception  of  the  atom  of  electricity,  the  foun- 
dation of  the  material  atoms,  evidently  enables  us  to 
penetrate  further  into  Nature's  secrets  than  our  pre- 
decessors ;  but  we  must  not  be  satisfied  with  words, 
and  the  mystery  is  not  solved  when,  by  a  legitimate 
artifice,  the  difficulty  has  simply  been  thrust  further 
back.  We  have  transferred  to  an  element  ever 
smaller  and  smaller  those  physical  qualities  which  in 
antiquity  were  attributed  to  the  whole  of  a  substance ; 
and  then  we  shifted  them  later  to  those  chemical  atoms 
which,  united  together,  constitute  this  whole.  To-day 
we  pass  them  on  to  the  electrons  which  compose  these 
atoms.  The  indivisible  is  thus  rendered,  in  a  way, 
smaller  and  smaller,  but  we  are  still  unacquainted 
with  what  its  substance  may  be.  The  notion  of  an 
electric  charge  which  we  substitute  for  that  of  a 
material  mass  will  permit  phenomena  to  be  united 
which  we  thought  separate,  but  it  cannot  be  con- 
sidered a  definite  explanation,  or  as  the  term  at 
which  science  must  stop.  It  is  probable,  however, 
that  for  a  few  years  still  physics  will  not  travel 
beyond  it.  The  present  hypothesis  suffices  for 
grouping  known  facts,  and  it  will  doubtless  enable 


326    THE  NEW  PHYSICS  AND  ITS  EVOLUTION 

many  more  to  be  foreseen,  while  new  successes 
will  further  increase  its  possessions. 

Then  the  day  will  arrive  when,  like  all  those 
which  have  shone  before  it,  this  seductive  hypothesis 
will  lead  to  more  errors  than  discoveries.  It  will, 
however,  have  been  improved,  and  it  will  have 
become  a  very  vast  and  very  complete  edifice  which 
some  will  not  willingly  abandon  ;  for  those  who  have 
made  to  themselves  a  comfortable  dwelling-place  on 
the  ruins  of  ancient  monuments  are  often  too  loth 
to  leave  it. 

In  that  day  the  searchers  who  were  in  the  van  of 
the  march  after  truth  will  be  caught  up  and  even 
passed  by  others  who  will  have  followed  a  longer, 
but  perhaps  surer  road.  We  also  have  seen  at  work 
those  prudent  physicists  who  dreaded  too  daring 
creeds,  and  who  sought  only  to  collect  all  the  docu- 
mentary evidence  possible,  or  only  took  for  their 
guide  a  few  principles  which  were  to  them  a  simple 
generalisation  of  facts  established  by  experiments ; 
and  we  have  been  able  to  prove  that  they  also  were 
effecting  good  and  highly  useful  work. 

Neither  the  former  nor  the  latter,  however,  carry 
out  their  work  in  an  isolated  way,  and  it  should  be 
noted  that  most  of  the  remarkable  results  of  these 
last  years  are  due  to  physicists  who  have  known 
how  to  combine  their  efforts  and  to  direct  their 
activity  towards  a  common  object,  while  perhaps  it 
may  not  be  useless  to  observe  also  that  progress 


THE  FUTURE  OF  PHYSICS  327 

has  been  in  proportion  to  the  material  resources  of 
our  laboratories. 

It  is  probable  that  in  the  future,  as  in  the  past,  the 
greatest  discoveries,  those  which  will  suddenly  reveal 
totally  unknown  regions,  and  open  up  entirely  new 
horizons,  will  be  made  by  a  few  scholars  of  genius  who 
will  carry  on  their  patient  labour  in  solitary  medita- 
tion, and  who,  in  order  to  verify  their  boldest  con- 
ceptions, will  no  doubt  content  themselves  with  the 
most  simple  and  least  costly  experimental  apparatus. 
Yet  for  their  discoveries  to  yield  their  full  harvest, 
for  the  domain  to  be  systematically  worked  and 
desirable  results  obtained,  there  will  be  more  and 
more  required  the  association  of  willing  minds,  the 
solidarity  of  intelligent  scholars,  and  it  will  be  also 
necessary  for  these  last  to  have  at  their  disposal  the 
most  delicate  as  well  as  the  most  powerful  instru- 
ments. These  are  conditions  paramount  at  the 
present  day  for  continuous  progress  in  experimental 
science. 

If,  as  has  already  happened,  unfortunately,  in 
the  history  of  science,  these  conditions  are  not 
complied  with;  if  the  freedoms  of  the  workers  are 
trammelled,  their  unity  disturbed,  and  if  material 
facilities  are  too  parsimoniously  afforded  them, — 
evolution,  at  present  so  rapid,  may  be  retarded, 
and  those  retrogressions  which,  by-the-by,  have  been 
known  in  all  evolutions,  may  occur,  although  even  then 
hope  in  the  future  would  not  be  abolished  for  ever. 


328    THE   NEW   PHYSICS  AND   ITS  EVOLUTION 

There  are  no  limits  to  progress,  and  the  field  of  our 
investigations  has  no  boundaries.  Evolution  will 
continue  with  invincible  force.  What  we  to-day  call 
the  unknowable,  will  retreat  further  and  further 
before  science,  which  will  never  stay  her  onward 
march.  Thus  physics  will  give  greater  and  increas- 
ing satisfaction  to  the  mind  by  furnishing  new 
interpretations  of  phenomena ;  but  it  will  accomplish, 
for  the  whole  of  society,  more  valuable  work  still, 
by  rendering,  by  the  improvements  it  suggests,  life 
every  day  more  easy  and  more  agreeable,  and  by 
providing  mankind  with  weapons  against  the  hostile 
forces  of  Nature. 


Index  of  Names 


ABRAHAM,  Prof.  Max.  187,  311, 

314. 
Academic  des  Sciences,  Comptes- 

rendus  of,  185. 
Alexeif,  114. 
Almeida,  d',  216. 
Amagat,  107,  111,  112,  115. 
Ames,  44. 

Ampere,  182,  183,  184,  214. 
Andrews,  105,  106. 
Annalen  der  Physik,  60,  196. 
Arago,  172,  221. 
Archives     Neerlandaises,     256, 

311. 
Armstrong,  Prof.  Henry,  185. 

Mr,  233. 
Arrhenius,  Prof.,  157,  159,  162, 

160-164,  250,  257. 
Association       frangaise       pour 

I'Avancement  des  Sciences, 

149. 

Athenceum,  the,  297. 
Atomic     Theory,     Development 

of  (Barlow  &  Pope),  318. 
Austen,  Sir  W.  Roberts,  141. 


BACH,  136. 
Balmer,  297. 
Barbillon,  190. 
Barkla,  Dr,  196. 
Barlow,  318. 
Bartoli,  194. 
Barus,  251. 
Batelli,  106,  151. 
Becquerel,   H.,    269,  274,   275, 
300,  304,  305. 


Becquerel,  Jean,  297. 
Becquerel   Rays,    The    (Strutt), 

275, 

Bell,  Graham,  219,  224,  229. 
Benoit,    Director    International 

Bureau     of     Weights    and 

Measures,  23,  25,  27. 
Benoist,  195. 

Bernouilli,  Daniel,  57,  58,  99. 
Berthelot,  Daniel,  38,  lo,   113 

116. 

Beschem,  Van,  230. 
Bessel,  47. 
Birkeland,  265. 
Bjerknes,  Prof.,  225. 
Blanc,  230. 
Bloch,  251. 
Blondel,  45. 
Blondlot,    189,    200-202,    225, 

308. 

Boltzmann,  40,  85,  100. 
Bonetti,  215. 
Bonn,  189. 
Borda,  47. 
Bordeaux,  216. 

Bose,  Prof.  J.  C.,  191,  225,  227. 
Boston,  220. 
Bouasse,  136,  137. 
Bouchot,  215. 
Bouguer,  24. 
Boulanger,  J.,  210,  211. 
Bourbouze,  216. 
Boussinesq,  174,  299,  300. 
Bouty,  162,  239. 
Bowman,  James,  217. 
Boyle,  58. 
Boys,  Vernon,  46. 


329 


330 


INDEX  OF   NAMES 


Branly,     Ed.,    224,    227,    229, 

230. 

Braun,  Ferdinand,  232,  233. 
Brillouin,  50,  102,  138. 
Briot,  299. 

Bristol  Channel,  217,  231. 
Broca,  197,  210,  304. 
Brooks,  Miss,  278. 
Brown,  C.  A.,  219,  224,  229. 
Brunhes,  197. 
Bucherer,  Dr  A.  H.,  172. 
Buisson,  33,  197. 


CAILLETET,  106,  115. 

Calzecchi-Onesti,  Prof.,  229. 

Carlisle,  4. 

Carlsruhe,  131. 

Carnot,  Sadi,  59,  60,  72-87, 
194,  290. 

Carvallo,  180,  299,  301. 

Cauchy,  93,  137,  299. 

Cavendish,  46. 

Cavendish  Laboratory,  247. 

Chappe,  Claude,  222. 

Chappuis,  P.,  38,  106,  115. 

Charles,  5. 

Charpy,  141. 

Chatelier,  le,  39,  141. 

Chemical  Society,  125. 

Chevalier,  139. 

Clairaut,  48. 

Clapeyrori,  117. 

Claude,  118,  119,  120. 

Clausius,  61,  72-87,  144,  158, 
307. 

Colardeau,  115,  197. 

Compan,  121. 

Congres  International  de  Phy- 
sique, Paris,  1900—10,210. 

Cooke,  217. 

Corbino,  305. 

Cornu,  10,  48,  304. 

Cosserat,  136. 

Cotton,  181,  301,  304,  305. 

Coulier,  243. 

Coulomb,  3,  137. 

Cremieux,  205,  206. 

Croll,  James,  204. 


Crookes,  Sir  William,  193,  198, 
225,  259-262,  266,  267, 
281. 

Curie,  Madame,  269-280. 

Pierre,  35,  54,  111,  121,  195, 
269-280,  285. 


DALTON,  110,  125. 

Danne,  278. 

Davy,  4,  59. 

Debierne,  273,  290. 

Decombe,  190. 

Deftbrges,  General,  47. 

Delambre,  24. 

Demanet,  45,  144. 

Democritus,  96. 

Dering,  G.  E.,  217. 

Desains,  216. 

Descartes,  10,  11,  12,  94,  169, 

203. 

Deslandres,  268,  297,  298,  304. 
Dessau,  Bernhard,  210. 
Deville,  Sainte-Claire,  78. 
De  Vries,  148,  150. 
Dewar,    Sir    James,    101,    li9, 

120-125,  279. 
Dolbear,  Prof.,  223. 
Donat,  215. 
Doppler,  103. 
Doubt,  49. 
Drude,  190,  254. 
Dufour,  Henri,  220. 
Duhem,  88,  92,  114,   138,  139 

140,  144,  198. 
Du      Laboratoire      d      VUsine 

(Houllevigue),  98, 
Dumas,  141. 
Dutrochet,  146,  147. 


EBERT,  49. 

Edison,  219,  221,  257. 
Electrons  (Lodge),  315. 
Elster,  Prof.,  206,  251,  292. 
Encyclopaedists,  the,  11. 
England,  225,  231. 
Eotvos,  Baron,  47. 
Erman,  237,  238. 


INDEX  OF   NAMES 


331 


Epicurus,  96. 

Essai    critique    sur    Vhypothese 
des  atomes  (Hannequin),  93. 
Euclid,  79. 


FABRY,  28,  176. 

Fahie,  J.  J.,  210. 

Faraday,    Michael,     158,     166, 

168,  169,  184,  185,  308. 
Faye,  194. 
Ferrie,  210,  211. 
Fery,  295. 

Fessenden,  Prof.,  233. 
First  Principles  (Spencer),  81. 
Fizeau,  26,  48,  222. 
Fortin,  265. 

Fortnightly  Review,  225. 
Foucault,  48,  222. 
Fourier,  221. 
France,  28,  225,  237. 
Franklin,  3. 
Fresnel,  93,  169-188,  222. 


GALILEO,  310. 

Galvani,  3. 

Ganot,  144,  156. 

Garbasso,  192. 

Gates,  Miss,  278. 

Gauss,  214. 

Gautier,  A.,  125. 

Gay-Lussac,  108,  278. 

Geitel,  Prof.,  206,  251,  292. 

General    Post   Office    (British), 

207. 

Geneva,  205. 
Germany,  225. 
Gibbs,  J.  Willard,  85,  88,  89, 

90,  91,  100,  114,  144,  145, 

164. 

Giese,  Prof.,  238,  239,  254. 
Giesel,  Prof.,  274,  281. 
Gilliland,  219. 
Gintl,  215. 
Godin,  24. 
Goldstein,  264,  275. 
Gottingen,  214. 
Gouy,'86,  195. 


Grenoble,  149. 
Griffiths,  43. 
Griineisen,  Prof.,  136. 
Guillaume,  C.  E.,  37,  140. 
Guldberg,  152,  164. 
Gutton,  190,  201. 


HAGA,  Prof.,  196. 

Hall  (Baltimore),  311. 

Hannequin,  93,  94,  95,  204. 

Hartmann,  Colonel,  136. 

Haiiy,  Abbe,  3. 

Heligoland,  233. 

Helmholtz,  von,  14,  61,  68,  88, 

89,  90,  168,  243,  300,  301. 
Henry,  220. 
Hertz,  Heinrich,  14,  160,  188, 

189,    190,    222,    223,    225, 

227,  233,  261,  302,  308. 
Hesehus,  Prof.,  256. 
Heydweiller,  Prof.,  53. 
Highton,  215. 
Him,  108. 
History  of  Wireless  Telegraphy 

(Fahie),  210. 
Hittorf,  259,  265. 
Hoff,  Prof,  van  t',  90,  145,  146, 

150,    152,    153,    155,    157, 

163,  166. 

Hoocke,  135,  136. 
Houllevigue,  97,  98,  266. 
Huber,  223. 

Hughes,  D.  E.,  224,  229. 
Hugoniot,  49. 
Hull,  194. 

INDIA,  225. 

Indian  Telegraph  Office,  216. 

International  Bureau  of  Weights 
and  Measures,  24. 

International  Congress  of  Elec- 
tricians, 1904,  45. 

Italy.  4,  225. 

JAGER,  151. 

Jahrbuch  d.  Radio  •  activitat, 
281. 


332 


INDEX  OF  NAMES 


Jamin,  9. 
Jeans,  256. 
Johnson,  216. 
Joule,  87,  129. 


KANT,  323. 

Kaufmann.  Prof.,  314. 

Kayzer,  Prof.,  298. 

Kelvin,  Lord,  11,   19,   61,   80, 

81,  173,  217,  287,  307. 
Ketchin,  125. 
Kirchhoff,     8,    40,    143,     144, 

294,  297,  306. 

Kohlrausch,  Prof.,  136,  162. 
Krigar-Menzel,  47. 
Kiinen,  114. 


LA  CONDAMINE,  24. 
Lafond,  Sigaud  de,  58. 
Lame,  170,  171. 
Lamotte,  190. 
Lampa,  191. 
Landholt,  Prof.,  53. 
Langevin,  71,  79,  95,  247,  248, 

252,  310,  321. 
Laplace,  5,   58,    59,    109,    171, 

204. 

Larmor,  Dr,  194,  302,  316. 
Lausanne,  220. 
Lavoisier,  51,  52,  53,  54,  58,  59, 

67. 

Lebedeif,  190,  193,  225. 
Le  Bon,  Gustave,  200,  270,  287, 

288,  289. 

Lecher,  190,  191,225. 
Lemons  eUnientaires  de  physique 

(Turpain),  180. 

Leduc,  Stephane,  44,  106,  149, 
Lee  de  Forest,  Dr,  233.      . 
Lehfeld,  114. 
Lehmann,  Prof.,  131. 
Lemme,  244. 
Lenard,   Prof.,   178,    197,    260, 

261,  263. 
Lenoble,  139. 
Lepinay,  Mace  de,  28,  33. 
Le  Royer,  230. 


Lesage,  205. 

Leucippus,  96. 

devolution   de  la,  matiere  (Le 

Bon),  270,  287. 
Leyden,    Physical    Laboratory, 

114. 

Liebig,  60. 
Linde,  117. 
Lindsay,  217,  219. 
Lippmann,  35,  86,  179. 
Lodge,  Sir  Oliver,  168,  225,  226, 

227,  230,  234,  315. 
Logemann,  Dr,  277. 
Loomis,  Mahlon,  223. 
Lorentz,    Prof.,   254,  256,  302- 

311,  320,  321. 
Lucretius,  96. 
Lummer,  Prof.,  41,  295. 
Lyons,  216. 


M'CLUNG,  196. 

Macaluzo,  305. 

Malus,  5. 

Mangin,  Colonel,  222. 

Marchis,  139. 

Marconi,   Guglielmo,   218,   224, 

226,    227,    228,    230,    231, 

232,  233,  234. 
Marx,  Prof.  E.,  196. 
Mascart,  201. 
Massieu,  88. 
Materiaux  pour  la  dynamique 

du  del  (Mayer),  60. 
Mathias,  106,  112,  116,  117. 
Matteucci,  Prof.,  251. 
Maxwell,    Clerk,  8.6,    100,  129, 

169,    185,    186,    187,    188, 

189,    193,   194,    205,    221, 

293,  308. 

Mayer,  Jules  Robert,  57,  60,  61. 
Mazotto,  Dominico,  211. 
Mecanique  celeste  (Laplace),  5. 
Melhuish,  W.  F.,  216. 
Mdmoire  sur  le  chaleur  (Laplace) , 

58,  59. 
Memoire     sur     le    mouvement 

organique  et    la    nutrition 

(Mayer),  60. 


INDEX  OF  NAMES 


333 


Mendeleeff,  172,  272. 

Meslin,  97,  111. 

Michelson,  Prof.,   27,   48,    103, 

176,  305. 
Ministry   of    Marine,    German, 

219. 

More,  151. 
Moreau,  250. 

Morse,  S.  F.  B.,  215,  234. 
Mouret,  76. 
Muller,  49. 
Munich,  214,  233. 


NANCY,  200. 

Nantes,  149. 

Natanson.  111. 

Nernst,    Prof.,   150,   161,   164- 

166,  200,  295. 
Neumann,  179,  180,  187. 
Newcomb,  48. 
Newton,  4,  97,   169,  182,  183, 

184,  203,  291,  309. 
New  York,  215. 
Nichols,  192,  194. 
Nicholson,  4. 
Niepce  de  St  Victor,  269. 
Nollet,  Abbe,  146. 
Notes  de  physique  experimental , 

(Demanet),  45,  144. 


OHM,  221,  241. 
Onnes,  Kamerlingh,  114. 
Optics  (Newton),  203. 
Osmond,  141. 

Ostwald,   Prof.,    67,    134,    162, 
164. 


PAINLEVE,  23,  310. 

Paris,  216. 

Parkinson,  119. 

Pellat,  253. 

Perot,  28,  117,  176. 

Perreau,  197. 

Pen-in,  J.,  70,  71,  79,  80,  246, 

262,  263. 
Pfeffer,  148,  150,  152. 


Philosophical  Dictionary  (Vol- 
taire), 234. 

Philosophical  Transactions,  247. 

Physics  (Ganot),  144,  156. 

Physikalische  Zeitschrift,  297. 

Planck,  Prof.,  40,  85,  134,  307. 

Pliicker,  265. 

Poincare,  H.,  126, 181, 185,  193, 
225,  301,  308,  310. 

Poisson,  137,  173 . 

Poissy,  216. 

Pope,  318. 

Popoff,  226,  227,  234. 

Poynting,  Prof.,  47,  54,  134, 
193. 

Preece,  Sir  William,  217,  218, 
227,  228,  234. 

Preston,  113,  304. 

Prevost,  Pierre,  5. 

Principles  of  Chemistry  (Men- 
deleeff), 172. 

Pringsheim,  Prof.,  41,  295,  296. 

Proceedings  of  the  Royal  Society, 
132,  277. 

QUINCKE,  Prof.,  97,  132. 

RADIO-ACTIVE  Transformations 

(Rutherford),  274,  281,284. 
Radio  •  Activity     (Rutherford), 

275  ;  (Soddy),  275. 
Ramsay,  Sir  William,  106,  123, 

124,  125,  279,  283,  284. 
Rankine,  65,  108. 
Rathenau,  Prof.  Emile,  219. 

W.,  219. 
Raoult,  145,  152. 
Raveau,  91,  112. 
Ray,  54. 

Rayleigh,  Lord,  125. 
Recent  Development  of  Physical 

Science  (Whetham),  275. 
Recknagel,  108. 
Reflexions     sur     la    puissance 

motricedufeu  (Carnot),  72. 
Reinold,  Prof.,  97. 
Remarques  sur  les  forces  de  la 

nature  animee  (Mayer),  60. 


334 


INDEX   OF  NAMES 


Richardson,  257. 

Richarz,  Prof. ,  47. 

Riecke,  254. 

Riemann,  49. 

Righi,Prof.,  190,  197,  210,  225, 

227,  241. 

Rive,  de  la,  190,  225. 
Rontgen,  Prof.,  7,  194,  199. 
Roozeboom,  Bakhuis,  90. 
Rothmund,  114. 
Rowland,  43. 
Royal  Institution,  120. 
Royal  Society,  122,  132,  277. 
Rubens,    Prof.,   176,   177,    191, 

192,  219. 

Rticker,  Sir  Arthur,  97. 
Rumford,  5,  6. 
Runge,  Prof.,  298. 
Russia,  225. 
Rutherford,  Prof.,  196,  226,252, 

270,    274,    275,    278,    281, 

282,  283,  284,  288. 
Rydberg,  Prof.,  298. 


SACHER,  219. 

Sagnac,  195,  206,  264,  276,  290. 

Sanchuniathon,  234. 

Sandford,  54. 

Sarrazin,  Prof.,  190,  225. 

Schmidt,  271. 

Schumann,  178,  198. 

Schwedoff,  129. 

Siemens,  118. 

Smith,  Willoughby  S.,  219. 

Soddy,    Prof.,    122,   275,    278, 

279,  281,  282,  283,  284. 
Sommerfeld,  Prof.,  196. 
Spencer,  Herbert,  81. 
Spezzia,  231. 

Spring,  W.,  101,  129,  130,  131. 
Stansfield,  141. 
Stark,  Prof.,  252,  297,  319. 
Steinheil,  Prof.  C.  A.,  214. 
Stephan,  295. 
Sterneek,  von,  47. 
Stevenson,  C.  A.,  219. 
Stewart,  Merrit,  197. 
Stokes,  Sir  George,  198,  244. 


Story,  211,  223. 

Story  of   Wireless  Telegraphy, 

The  (Story),  211. 
Strasbourg,  232,  233. 
Stmtt,  Hon.  R.  J.,  275. 
Stuttgart,  Naturforscherver- 

sammlung,  172. 
Sur  la  base  du  systeme  metrique 

decimal  (Delambre),  24. 
Sutherland,  200,  321. 
Switzerland,  225. 

TAINTER,  Summer,  229. 

Tait,  101. 

Tamman,  132,  133,  134. 

Telegraphie  ohne  Draht  (Des- 
sau), 210. 

Telegraphie  sans  fil  (Broca), 
210;  (Mazotto),  211. 

Telegraphie  sans  fil  et  les  ondes 
electriqucs  (Boulanger  et 
Ferrie),  210,  211. 

Tesla,  transmission  of  oscilla- 
tions, 226. 

Theory  of  Heat  (Preston),  113. 

Thiele,  298. 

Thomson,  Prof.  J.  J.,  199,  239, 
244,  245,  254,  257,  266, 
311. 

Thot,  234. 

Threlfall,  Prof.  R.,  225. 

Tissot,  232. 

Toise  du  Chatelet,  23,  24. 
du  Nord,  24. 
du  Perou,  24. 

Tour,  Cagniard  de  la,  105. 

Transactions  of  the  Chemical 
Society— 1906,  318. 

Travers,  Dr,  123,  126. 

Traube,  Prof.,  148,  151. 

Tresca,  128. 

Trowbridge,  Prof.,  220. 

Turpain,  180,  190. 

UNITED  STATES,  215. 

VARLEY,  229. 
Vautier,  49. 


INDEX   OF   NAMES 


335 


Verdet,  Prof.,  9,  64,  173. 

Vienna,  196,  219. 

Villard,  149,  197,  265,  268,  275. 

Vincent,  97. 

Violle,  10,  45,  4P. 

Voigt,  137,  305. 

Volta,  3. 

Voltaire,  234. 


WAAGE,  152,  164. 

Waals,  van   der,  90,  108,   109, 

110,  111,  113,  114,  116. 
Weber,  W.,  214,  254. 
Wheatstone,  215,  217. 
Whetham,  275. 
Wiechert,  Prof.,  198,  267. 
Wiedermann,  E.,  259. 
Wien,  Prof.,  40,  268,  295,  307. 


Wiener,  98,  179,  180,  193. 
Wilkins,  S.  W.,  215,  217. 
Wilsing,  47. 
Wilson,  C.  T.  R.,  243,  244,  245, 

250. 

Wind,  Prof.,  196. 
Winterson,  Prof.,  125. 
Wireless  Telegraphy,  The  Story 

of  (Story),  211. 


YOUNG,  Thomas,  4,  6,  59. 
Mr  S.,  106,  112,  115,  222. 


ZEEMAN,  Prof.,  303,  305. 
Zeleny,  Mr,  247.     . 
Zerdust,  234. 


Index  of  Subjects 


ABSOKBABLE  rays,  178,  198. 

Absorption,  phenomena  of,  306. 

Absorption  and  emission,  pro- 
portion between  power  of, 
294,  297,  306. 

Actinium, discovery  by  Debierne, 
273  ;  rays,  277  ;  radio-ac- 
tinium, 282. 

Actinium  X,  discovered  by 
Giesel,  281. 

Action,  principle  of  equality  of, 
and  reaction,  51. 

Air,  liquid,  distillation  of 
oxygen  by,  119  ;  recovery  of 
cold  produced  by,  119  ; 
Claude's  "reversed  method," 
119,  120. 

Alloys,  solid  and  liquid,  141  ; 
variations  in  reaching  equi- 
librium, 142  ;  theory  of  the 
eutectic  point,  145. 

Alternating  currents,  communi- 
cation by,  219. 

Antennae,  role  of  the,  232. 

Argon,  123,  124,  125. 

Assimilation  between  electrical 
and  luminous  phenomena, 
222. 

Atmosphere,  rare  gases  of,  Sir 
William  Ramsay's  discovery, 
123,  279  ;  Dr  Travers,  123  ; 
Sir  James  Dewar,  124,  125  ; 
separation  of,  124  ;  mono- 
atomic,  124  ;  probable  testi- 
mony to  vanished  conditions 
of  earth,  124 ;  distribution 
of,  125. 


Atoms,  ejection  of  electron  by, 
287,  307  ;  atomism,  92-104 ; 
essay  of  Hannequin,  93  ; 
atomic  theory  and  optics, 
heat,  chemistry,  93  ;  pre- 
ponderating part  of,  in 
physics,  95  ;  to  be  promoted 
from  rank  of  hypothesis  to 
that  of  principles  (Langevin), 
95  ;  atomism  not  to  be  con- 
fused with  molecular  physics, 
95  ;  evolution  of  ideas  con- 
cerning, 96  ;  properties  of  a 
body  modified  in  small  mass, 
97  ;  atomistic  synthesis  now 
accepted,  103  ;  complex  edi- 
fices formed  of  elements,  104  ; 
electric,  suggestion  of,  168, 
246  ;  not  the  smallest  fractions 
of  matter,  249  ;  metallic, 
dissociation  of,  254  ;  of  hy- 
drogen, 257,  315  ;  dissociation 
of  material,  yields  electrons, 
268  ;  atomic  energy  and  disso- 
ciation of  matter,  282  ;  atoms 
of  radio-active  bodies  easily 
detached,  282  ;  and  radium, 
284  ;  complete  dislocation  of, 
288  ;  supposition  concerning 
dynamical  system,  305  ;  com- 
position of  neutral,  306  ;  posi- 
tively charged  centre  of,  318  ; 
of  the  stars,  321. 

Auer-Welsbach  gas  mantles, 
295. 

Aurora  borealis,  spectrum  of, 
125. 


336 


INDEX  OF  SUBJECTS 


337 


BATTERY,  galvanic,  2,  3. 
Brownian  movements,  86. 

CANAL  rays,  see  Rays. 

Capillarity,  97  ;  theory  of,  109. 

Cathode  rays,  see  Rays. 

Cells,  foam,  132  ;  artificial,  149. 

Chemical  equilibrium,  Guldberg 
and  Waage's  law,  152. 

Chronometry,  progress  of,  36. 

Colour  photography,  and  sta- 
tionary waves,  179. 

Condensation,  phenomena  of 
retrograde,  115 ;  of  water- 
vapour  by  ions,  242-249 ; 
experiments  by  Coulier, 
Helmholtz,  Wilson,  Thom- 
son, 243-245. 

Conductivity  of  gases,  235-242  ; 
value  of  study  of,  236  ;  mis- 
taken ideas  of,  237  ;  work  of 
Erman,  Giese,  237,  238  ;  X- 
ray  experiment,  239  ;  calorific, 
due  to  exchange  of  electrons, 
255,  256. 

Conductivity  of  metals,  254. 

Conservation-  of  energy,  and 
mechanics,  61. 

Constants,  measurement  of 
physical,  45-50  ;  critical,  111, 
114,  115. 

Corresponding  states,  discovery 
of,  110;  laws  of,  114. 

Cryoscopy,  163. 

Crystals,  Lehmann's  liquid, 
131  ;  observed  only  under 
microscope,  131  ;  intermedi- 
ary forms  between  crystals 
and  liquids,  132  ;  Tamman's 
view  of  discontinuity,  134  ; 
Voigt's  researches  on  elasticity 
of,  137,  138. 

Current  of  saturation,  241. 

DEFORMATION  of  solids  and 
liquids,  128,  129,  135-142; 
Hoocke's  law  governing 
elastic,  135 ;  researches  of 
Bouasse,  136,  137  ;  and  mole- 


cular hypothesis,    138 ;    and 
theory     of    thermodynamics, 

139  ;     verification     of,     139, 

140  ;    experiments   on   nickel 
steel,     140;     application     to 
alloys,  141,  142. 

Dissociation,  electrolytic,  155- 
168  ;  defined,  156  ;  researches 
of  Van  t'Hoff,  ]  55-157  ;  ionic 
hypothesis  of  Arrhenius,  157, 

159,  162  ;  opposition  to,  159, 

160,  161  ;  advantages  of,  161  ; 
Faraday's  laws  of,  166,  168  ; 
and    valency,    167  ;    experi- 
ments of  Sir  Oliver  Lodge  in, 
168. 

Dust  and  germs,  243,  257. 

ELASTICITY  of  solids,  135,  136, 
137;  of  crystals,  137. 

Electric  wave  experiments,  192. 

Electrical  and  magnetic  actions, 
reciprocity  between,  188,  189. 

Electrical  phenomena,  9. 

Electrical  waves,  225,  226,  227, 
230,  234. 

Electricity,  and  mechanics,  17  ; 
composed  of  elementary  parts 
(Helmlioltz),  168  ;  suggestion 
of  electric  atom,  168,  246 ; 
and  optics,  221,  293,  308  ; 
dissymmetry  of  positive  and 
negative,  237  ;  and  condensa- 
tion of  water- vapour,  243. 

Electrified  body,  inertia  of,  311. 

Electrolysis,  phenomena  of,  158, 
164,  236,  250,  257;  theory 
of  Arrhenius,  162  ;  Nernst's 
theory  of,  164-166  ;  dissym- 
metry revealed  by,  238. 

Electrolytes,  158  ;  conductivity 
of,  159,  162  ;  diffusion  of,  164. 

Electromagnetic  phenomena, 
possibility  of  mechanical  ex- 
planation of,  186-189,  194, 
205  ;  theories  of  Maxwell  and 
Hertz,  188,  193,  293  ;  theories 
of  dispersion,  301 ;  theory  of 
Langevin,  310,  321. 

22 


338 


INDEX  OF  SUBJECTS 


Electrons,  6,  249,  305  ;  in 
metals,  253-257  ;  smaller  than 
atoms,  254 ;  movements  of, 
254,  255;  identical  in  all 
metals,  255  ;  and  electrifica- 
tion by  contact,  256  ;  kinetic 
energy  of,  256  ;  emission  of, 
by  charged  body,  257 ; 
Thomson's  researches  on,  - 
254-257 ;  his  measurement 
of  positive  ions,  257  ;  and 
cathode  rays,  266 ;  ejection 
of,  by  radio-active  atom,  287, 
307  ;  hypothesis  of,  results, 
293  ;  mobile,  sensitive  to 
magnet,  303  ;  theory  of 
Lorentz,  303  seq.  ;  phenomena 
of  emission,  306;  of  absorption, 
306  ;  mass  of,  311-316  ;  Hall's 
experiment,  311  ;  longitudinal 
and  transversal  mass,  313  ; 
speed  of,  cannot  exceed  that 
of  light,  313,  314;  ratio  of 
charge  to  mass,  314  ;  a  simple 
electric  charge,  315  ;  and  pro- 
duction of  waves,  316,  and 
Rontgen  rays,  316,  319;  is 
matter  all  electrons?  317; 
present  supremacy  of  electron 
theory,  "  the  new  idol,"  324. 

Emission  and  absorption,  pro- 
portion between  power  of, 
294,  297,  306. 

Energetics,  Mayer,  61  ;  Ran- 
kine's  phrase,  65,  108,  289. 

Energies,  classification  of,  de- 
gradation, 83. 

Energy,  measure  of,  42-45  ;  erg 
as  unit,  43  ;  calorimeter  usual 
means  of  determination,  44  ; 
photometric  units,  44  ;  prin- 
ciple of  conservation  of.  55- 
72 ;  work  of  Bernouilli, 
Lavoisier,  Laplace,  Davy, 
Sadi  Carnot,  Mayer,  57-61  ; 
efforts  to  introduce  principle 
into  mechanics,  61,  62  ;  con- 
stant relation  between  quan- 
tities of  work  and  heat,  63  ; 


Verdet's  predictions,  64,  65  ; 
usefulness  of  work  as  standard 
form  of,  66,  68  ;  objections, 
68  ;  matter  form  of  energy, 
68  ;  importance  of  principle 
of  conservation,  70-72  ;  heat 
end  of  all  energy  (Kelvin, 
Clausius),  81  ;  use  of  terms, 
available,  free,  88,  bound, 
90,  114  ;  Faraday's  views  on 
electrical,  185;  atomic,  and 
disaggregation  of  matter,  282- 
292  ;  liberated  by  radium, 
284,  285  ;  is  source  of,  outside 
the  atom  ?  290  ;  gravitational, 
290,  291. 

Entropy,  result  of  Clausius' 
postulate,  75  ;  a  variable,  77  ; 
incessant  increase  (Clausius), 
78,  82  ;  Kelvin,  80,  81  ;  a 
property  added  to  matter,  81  ; 
variation  of,  takes  character 
absolute  certainty,  85,  96, 
145  ;  Clausius,  158,  307. 

Erg,  the,  43,  44,  45. 

Ether,  the,  169-207  ;  lumini- 
ferous,  169-175  ;  ideas  of 
Descartes,  1 69  ;  growth  of 
idea,  169 ;  Fresnel's  hypo- 
thesis of  luminous,  170,  222  ; 
Lame  on  the  existence  of  the, 
171  ;  of  transverse  vibrations, 
172,  173;  resembles  a  solid 
(Kelvin),  173,  174;  discon- 
tinuous medium,  175  ;  radia- 
tions in,  175,  182  ;  density 
same,  elasticity  variable,  180  ; 
kinetic  energy  in  photo- 
graphy, 181  ;  electromagnetic, 
182-189;  Maxwell  and  elec- 
tromagnetic waves,  185-189  ; 
electrical  oscillations,  189- 
194  ;  Hertz' experiments,  189  ; 
identification  of  electromag- 
netic and  luminous  waves, 
190 ;  and  gravitation,  203- 
207  ;  ideas  of  Descartes  and 
Newton,  203,  204  ;  velocity  of, 
204  ;  experiments  of  Cremieux, 


INDEX  OF  SUBJECTS 


339 


205,  206  ;  has  it  an  objective 
existence  ?  207 ;  luminous, 
and  transmission  of  signals, 
218 ;  and  matter,  relations 
between,  293-302 ;  laws  of 
radiation,  295 ;  theory  of 
Lorentz,  302-311  ;  new  views 
on  the  construction  of,  316- 
321  ;  definable  by  electric  and 
magnetic  fields,  317. 

Equilibrium,  elastic,  and  thermo- 
dynamics, 89;  of  system,  91, 
92,  139. 

Equivalence,  principle  of,  57,  59, 
64,  73. 

Eutectic  point,  145. 

FLUIDS,  statics  of,  105-117  ; 
meaning  of,  105 ;  work  of 
Amagat,  107 ;  equation  of 
Van  der  Waals,  108-110; 
result  of,  discovery  of  corre- 
sponding states,  110,  114, 

116  ;     Amagat's     superposed 
diagrams,  111,  112  ;  statics  of 
mixed  fluids,  113, 115;  critical 
constants,  115  ;  characteristic 
equation  not  yet  known,  116. 

Fluorescence,  visibility  of,  101. 

GAS,  incandescent  lighting,  295. 

Gases,  kinetic  theory  of,  57,  58, 
98,  99  ;  conductivity  of,  103, 
235-242  (see  Conductivity) ; 
liquefaction  of,  at  low  tempera- 
tures, 117-126;  two  categories, 

117  ;    experiments  of  Linde, 
Siemens,    Claude,    ' '  reversed 
method,"  117-120;  Sir  James 
Dewar's  researches,  120,  121, 
122,  125;  Dalton's  law,   125; 
modification  of  electrical  and 
mechanical       properties      of 
matter     by    excessive    cold, 
120,    121;  creation  of  vacua, 
122 ;   Ramsay's    discovery   of 
rare,     in     atmosphere,     123; 
monoatomic,    124;  their  dis- 
tribution, 124  ;  liquid  hydro- 


gen, 125,  126;  gases  and  ions, 
235-257  (see  Ions);  passage  of 
electricity  through,  237;  char- 
acteristics of  ionised,  241; 
electrified  centres  in,  244. 
Gravitation,  constant  of,  36, 
49  ;  measurement  of  intensity, 
47  ;  is  action  of  due  to  oscilla- 
tions ?  204  ;  Newton,  182, 
183,  184,  291,  309;  diver- 
gences between,  and  other 
phenomena  more  apparent 
than  real,  205  ;  Lorentz,  320, 
321. 

HEAT,  nature  of  (Rumford),  5, 
6 ;  and  work,  57,  63,  64 ; 
definition  of  Lavoisier  and 
Laplace,  58  ;  on  (Sadi  Carnot). 
59,  60,  79,  80,  194,  290; 
principle  of,  72-87 ;  energy 
manifested  in,  90  ;  theory  of 
(Preston),  H3 ;  and  vapours 
of  gases,  117. 

Helium,  123,  134,  125,  126, 
283,  284  ;  discovery  of,  279. 

Hydrogen,  liquid,  123;  diffi- 
culty of  liquefying,  126  ; 
specific  heat  greatest  known, 
126  ;  not  conductor  of  electri- 
city, 126 ;  charge  of  an  ion 
of,  167. 

Hypothesis,  atomic,  106  ;  bal- 
listic, 289. 

ICELAND  spar,  91. 

Induction  phenomena,  221. 

Inertia  of  electrified  body,  311. 

lonoplasty,  266. 

Ions,  Faraday's  term,  158  ;  two 
kinds,  158,  245  ;  Arrhenius' 
hypothesis,  159  ;  Nernst's 
theory,  161  ;  course  of  positive 
and  negative,  162,  240  ;  "the 
ions  which  react,"  163  ;  metal- 
lic, 165  ;  electrolytic,  166  ; 
Faraday's  laws,  166,  167  ; 
charge  of  an  ion  of  hydrogen, 
167  ;  speed  of,  167,  168,  240  ; 


340 


INDEX   OF  SUBJECTS 


conductivity  of  gases  and  the, 
235-257;  ions  and  conduction 
in  gases,  238 ;  condensation 
of  water -vapour  by,  242-249  ; 
twenty  million  ions  per  centi- 
metre of  gas,  245  ;  neutral, 
245;  condensation  of  positive 
and  negative,  245  ;  charge 
borne  by,  246  ;  gaseous  not 
identical  with  electrolytic, 
246  ;  velocities,  247  ;  Zeleny's 
measurements,  247 ;  Lange- 
vin's  experiments,  247,  248, 
252  ;  production  of,  249-253  ; 
experiments  with  alkaline 
salts,  250  ;  with  phosphorus, 
251  ;  considerable  energy 
necessary  for  production  of, 
252 ;  ionisation  result  of 
shock,  252  ;  Thomson's  meas- 
urement of  positive,  257  ; 
positive,  268  ;  theory  of,  takes 
preponderant  place,  324. 

Iron,  reactions  of,  164. 

Isothermal  diagrams,  106. 

KILOGKAMME,  international,  33. 
Kinetic  energy,  101,  256;  theory 

of,  58,    101,    102,    103,    116, 

167,  245,  257. 
Kinetics  and  solution,  143. 

LANTHANUM,  273. 

Law,  of  corresponding  states, 
110,  111,  112,  114  ;  of  elec- 
trolysis, 98  ;  Faraday's,  166, 
168  ;  Gibbs',  164  ;  Guldberg 
and  Waage's,  152,  164; 
Hoocke's,  135,  136 ;  Mari- 
otte's,  100,  108,  157,  184, 
278  ;  of  multiple  proportions, 
98  ;  Ohm's,  241  ;  phase,  90, 
91,  145,  152;  Raoult's,  152. 

Length,  measure  of,  21-30  ; 
idea  of  absolute,  22  ;  history 
of  standard,  23,  24  ;  standard 
metre,  27,  30  ;  agreement 
between  legal  and  scientific 


definition  of  metre,  28  ;  unit, 
of,  36. 

Light,  polarisation  of,  5  ;  speed 
of,  48,  49  ;  synthesis  of,  192  ; 
pressure  of,  194.  (See  Ether, 
Rays,  Waves.) 

Liquids  and  solids,  126-134  ; 
viscosity  of,  128,  129  ; 
rigidity  of,  128,  129  ;  Spring's 
analogies,  130 ;  Lehmann's 
liquid  crystals,  131  ;  Tarn- 
man's  views  on  discontinuity, 
132,  134. 

Luminescence  and  incandescence, 
296,  297. 

MAGNETIC  phenomena,  184, 
186  ;  properties  depend  on 
state  of  aggregation  of  mole- 
cules, 121. 

Magnitude,  expression  by  num- 
ber (Kelvin),  19  ;  magnitudes, 
study  of  relations  of,  14  ;  of 
electricity  and  mechanics,  64. 

Malacone,  125. 

Mariotte,  law  of,  100,  108,  157, 
184,  278. 

Mass,  measure  of,  31-33  ;  dis- 
tinction between,  and  weight, 

31  ;  kilogramme  as  standard, 

32  ;    researches  of    Mace    de 
Lepinay    and    Buisson,    33  ; 
precision  in  comparison,  33  ; 
future  improvements,  33 ;  unit 
of,  36  ;  is  it  indestructible  ? 
52. 

Mathematical  expression,  advo- 
cacy of,  182-184. 

Matter,  indestructibility  of,  52  ; 
radioactive  bodies  and  the 
disaggregation  of,  53  ;  dis- 
continuity of,  96,  98  ;  final 
division  of,  103 ;  various 
states  of,  105-117  ;  statics  of 
fluids,  105-117  (see  Fluids)  ; 
liquefaction  of  gases,  117-126 
(see  Gases) ;  deformation  of 
solids  and  liquids,  135-142 
(see  Deformation) ;  molecular 


INDEX  OF  SUBJECTS 


341 


constitution  of,  235 ;  dis-  I 
aggregation  of,  and  atomic 
energy,  282-292  ;  transforma- 
tions of,  in  radioactive  bodies, 
283,  284  ;  relations  between 
the  ether  and  matter,  293- 
302  ;  attempts  to  reduce  all 
matter  to  forms  of  ether, 
293  ;  reciprocal  actions  and 
reactions  between,  294  ; 
theory  of  radiation,  294 ; 
researches  of  Cauchy,  Bous- 
sinesq,  Poincare,  Helmholtz, 
299-302  ;  successive  trans- 
formations of,  probable,  320. 

Measurements  (Lord  Kelvin), 
19  ;  of  length,  mass,  time, 
temperature,  energy,  physical 
constants,  19-50  ;  science  of, 
20 ;  instruments  of,  25  ;  (Prof. 
Michelson),  27,  48,  103,  176, 
305 ;  by  torsion  balance  (Cav- 
endish). 46;  (Prof.  Poynting), 
47,  54,  134,  193  ;  of  physical 
constants  (Stephane  Leduc), 
44,  106;  of  mobilities  (Zeleny), 
247  ;  of  temperature  (Prof. 
Wien),  295. 

Mechanics,  the  study  of  revers- 
ible phenomena,  13  ;  is 
mechanics  a  branch  of  elec- 
trical science?  17,  18;  and 
conservation  of  energy,  61  ; 
and  energy,  61,  68,  88,  89  ; 
magnitudes  of,  64  ;  an  experi- 
mental science,  96. 

Metals,  electrons  in,  253-257 
(see  Electrons)  ;  conductivity 
of,  254. 

Metre,   history  and   description 
of  standard,  23-28  ;  legal  and 
scientific  definition  of,  28-30  ;    j 
international,  30. 

Metrical  system,  law  of  July 
11,  1903,  28,  30,  33  ;  in- 
tention of  founders,  32. 

Metrology,  19-21  ;  Lord  Kel- 
vin's view,  19  ;  definition  of, 
20,  21  ;  33  ;  progress  of,  45. 


Micrometer,  light  ray  as,  26. 

Microphone,  the,  229. 

Mixture,  binary,  of  fluids,  has 
critical  space,  not  critical 
point,  115. 

Molecules,  linear  dimensions, 
98  ;  grouping  of,  99  ;  free 
path  of,  102  ;  kinetic  theory 
of,  103  ;  saline,  158,  159"; 
dissociation  of,  159  ;  neutral, 
249 ;  of  the  ether  and  of 
matter,  296  ;  molecular  phy- 
sics, 95,  96  ;  molecular  consti- 
tution of  matter,  235. 

NEODYNIUM,  273. 

Neon, 

Nernst  lamp,  200,  295. 

Nitrogen,  liquefaction  at  high 
altitudes,  125. 

N-rays,  200-202  ;  discovery  by 
Blondlot,  200  ;  supposed  wave- 
lengths, 202  ;  value  of,  202. 

OHM,  law  of,  not  valid  for  ionised 
gases,  241. 

Optics,  now  part  of  electricity, 
188  ;  Newton,  203;  bridge 
between,  and  electricity,  221, 

.   222,  293,  308. 

Oscillations,  electrical,  189-194  ; 
experiments  of  Hertz,  189  ; 
electromagnetic  disturbance 
propagated  with  speed  of 
light,  189 ;  waves  between 
Hertzian  and  visible,  190  ; 
contrasts,  191-193  ;  pressure 
of  light,  1 93  ;  transmission  of 
(Tesla),  226. 

Osmosis,  145 ;  phenomena  of, 
146-151  ;  daftned,  146;  ex- 
periment of  Dutrochet,  146  ; 
semi- permeable  walls,  147- 
149  ;  pressure,  150  ;  direction 
and  speed  of,  151  ;  applica- 
tion to  theory  of  solution, 
151-155  ;  laws  of,  163. 

Oxygen,  Claude's  experiments, 
119,  120  :  magnetic  suscepti- 


342 


INDEX  OF  SUBJECTS 


bility   of,    121  ;   liquid,    122, 
125. 


•PHASE  law,  90,  91,  145,  152. 

Phosphorescence,  123,  269. 

Phosphorus  and  ions,  251. 

Photography  in  colour,  179  ; 
photographic  substances  and 
sensitiveness  to  light,  122  ; 
mechanism  of  photographic 
impression,  181  ;  salts  of 
uranium  and  photographic 
plates,  269. 

Physical  world,  co-ordination  of, 
and  hypothesis  of  electrons, 
293.  " 

Physics,  evolution  of,  1-18  ; 
research  in  1800-1810,  2-5; 
evolution,  not  revolution, 
6-8  ;  and  mechanics,  9-13  ; 
methods  of  modern  workers 
in,  14-18  ;  principles  of,  51- 
55  ;  conservation  of  energy, 
55-72 ;  principle  of  Carnot 
and  Clausius,  72-87  ;  thermo- 
dynamics, 87-92  ;  atomism, 
92-104  ;  future  of,  322-328  ; 
ambition  to  discover  supreme 
principle  in,  323  ;  need  for 
association  and  solidarity  of 
scientific,  327. 

Polarisation,  phenomena  of, 
132,  172  ;  Arago's  experiment, 
172  ;  rotatory,  301. 

Polonium,  273,  277. 

Pressure,  experiments  ( Amagat), 
107,  111,  112,  115;  internal, 
155  ;  manometric,  154,  155  ; 
osmotic,  150-155  ;  165,  166. 

Principles,  see  Physics. 

Properties  of  bodies  at  low 
temperatures,  117-126. 


RADIO-ACTINIUM,  282. 

Radioactive  bodies,  258-292 ; 
cathode  rays,  258-268  ;  sub- 
stances, 269-274  ;  uranic  rays, 


268-270  ;  radium,  271-273  ; 
actinium,  273 ;  emanation 
and  radiations,  274-282 ; 
researches  of  Giesel,  Bec- 
querel,  Rutherford,  274  ;  a, 
ft,  7  rays,  274-277  ;  induced 
radio-activity  from  radium, 
277,  278;  properties  of  emana- 
tions, 278  ;  transformations 
of  radioactive  bodies,  279- 
282  ;  ejection  of  electron  by, 
287,  307  ;  emission  of  radia- 
tions, explanation  of  pheno- 
mena of,  306. 

Radiations,  ultra-violet,  178, 
197. 

Radio-conductors,  discovery  of, 
228-230. 

Radiophony,  discovery  of,  by 
Bell,  224,  225. 

Radio-thorium,  282. 

Radium,  discovery  of,  5,  269- 
272  ;  researches  of  M.  and 
Madame  Curie,  269-280,  285  ; 
principle  of  concentration 
simple,  its  application  labori- 
ous, 272  ;  spectrum  of,  272  ; 
atomic  weight  of,  272 ; 
chemical  and  physiological 
actions  of,  272,  273  ;  possesses 
large  proportions  of  o,  ft,  and 
7  rays,  277  ;  radioactivity 
induced  by,  277  ;  bromide  of 
radium,  279  ;  transformation 
of,  into  helium,  283  ;  dis- 
appearance of  one-half  in  1280 
years,  284  ;  energy  liberated 
by,  285  ;  explanation  of  its 
radioactivity,  287  ;  borrowed 
energy  of,  290 ;  Sagnac's 
experiments,  290,  291. 

Rays,  a,  ft,  7,  274,  275,  276, 
277. 

Absorbable,  178,  198. 
Canal    of    Goldstein,    264, 
275,  318,  319. 

Cathode,  199,  235,  258- 
268,  302,  308  ;  history  of 
discovery  of,  258  ;  Crookes' 


INDEX   OF   SUBJECTS 


343 


theory,  259  ;  discovery  of 
Lenard,  260,  261;  Hertz,  261, 
302,  308  ;  convection  of 
negative  electricity  by,  262  ; 
experiment  of  Perrin,  262, 
263 ;  are  electrons  in  rapid 
motion,  263  ;  give  rise  to  X- 
rays,  264  ;  canal  rays.  264  ; 
Villard's  researches,  265,  268  ; 
ionoplasty,  266  ;  Thomson's 
measurements  of  speed  of,  266, 
267,  313 ;  dissociation  of  all 
atoms,  268. 

N-,  discovery  of,  at  Nancy 
by  Blondlot,  200  ;  doubts 
concerning,  201  ;  supposed 
wave  -  lengths,  202  ;  wave- 
lengths, 225,  308. 

Secondary,  of  Sagnac,  264, 
276. 

Uranic,  discovery  by  Bec- 
querel,269,270,  274,  275,  300, 
304,  305. 

X-,  5,  7,  194-202  ;  Ront- 
geri's  discovery,  194  ;  proper- 
ties of,  195  ;  Sagnac,  re- 
searches on,  195,  206;  energy 
corresponding  to,  196  ;  polari- 
sation of,  196  ;  speed  that  of 
light,  196  ;  whether  ultra- 
violet? 197  ;  theories  of  Stokes 
and  Wiechert,  Thomson,  Le 
Bon,  Sutherland,  198-200  ; 
experiment  with,  239,  241, 
243  ;  ionisation  by,  246,  250, 
270,  319. 

Reciprocity  between  electrical 
and  magnetic  actions,  188, 
189. 

Rigidity,  of  matter,  128,  129  ; 
of  the  ether,  173. 

SALTS,  dissociation  of,  163. 

Saturation,  current  of,  241. 

Scale,  thermometric,  38. 

Science  and  atomism,  93. 

Senses,  only  windows  of  external 
reality,  12;  everything  appar- 
ent to,  is  energy,  67. 


Signals,  transmission   of  (Lord 

Kelvin),  217;   (Popoff),    226, 

227,  234. 

Soap-bubble,  dark  zone  on,  97. 
Solids  and  liquids,  126-134  (see 

Liquids) ;     deformations     of, 

135-142  (see  Deformations). 
Solutions,       and       electrolytic 

dissociation,      143-168      (see 

Dissociation). 
Solvents,  temperature  of,    145, 

152. 

Space,  critical,  115. 
Spark-gap,  190. 
Spectrum,  constitution  of,  297  ; 

of  aurora  borealis,  125. 
Spinthariscope,   the    (Crookes), 

266,  267,  276,  281. 
Standard,  calorific,  43. 
States,  corresponding,  discovery 

of,  110. 
Statics,  of  fluids,  105-117   (see 

Fluids)  ;  thermodynamic,  89, 

92. 
Stationary  waves  (Wiener),  179, 

180,  193  ;  and  colour  photo- 
graphy   (Lippmann),      179  ; 

(Cotton),  181,  301,  304,  305. 
Steel,  Guillaume's  experiments 

on  nickel,  140. 

TELEGRAPHY,  wireless,  208- 
234  ;  histories  of,  208-211  ; 
two  systems  of,  211-213  ; 
experiments  by  d'Almeida, 
Steinheil,  Morse,  Johnson, 
Melhuish,  Wheatstone,  214- 
217 ;  Preece's  results  across 
Bristol  Channel,  217,  218  ; 
his  support  of  Marconi, 
218  ;  early  attempts  at  trans- 
mission of  messages  through 
ether,  182-220  ;  forerunners 
of  ether  telegraphy,  220-225  ; 
Hertzian  waves,  225 ;  work 
of  Threlfall,  Crookes,  Tesla, 
Lodge,  Rutherford,  Popoff, 
225-227  ;  Marconi's  practical 
system,  227  ;  the  receiver, 


344 


INDEX  OF  SUBJECTS 


228  ;  explanation  of  coherer 
still  obscure,  230  ;  commercial 
stage  of,  230  ;  defect  of 
Marconi's  system,  232  ;  use  of 
earth,  232,  233  ;  Hertz  and 
Marconi  take  premier  posi- 

.  tion  in,  233. 

Temperature,  measure  of,  37- 
41  ;  units  of,  37  ;  divergence 
of  thermometric  and  thermo- 
dynamic  scales,  38  ;  researches 
of  Berthelot,  38,  39,  40  ;  uses 
of  helium  thermometers,  39  ; 
improvements  in  thermo- 
metry,  41  ;  properties  of 
bodies  at  low,  117-126  ; 
measurement  of,  295. 

Thermodynamics,  in  regard  to 
measurements,  38-41,  87- 
92  ;  work  of  Massieu,  Gibbs, 
Helmholtz,  Duhem,  88  ; 
statics  of,  89,  139  ;  phase 
law,  90-92,  106,  116,  117  ; 
and  solution,  143  ;  second 
principle  of,  144. 

Thermometric  scale,  38. 

Theorem  on  entropy  (Clausius), 
78,  144,  158,307. 

Theory,  of  capillarity  (Laplace), 
109  ;  of  heat  (Preston),  113  ; 
of  equations  of  thermo- 
dynamics (Duhem),  139  ;  of 
electrolysis  (Nernst),  164- 
166  ;  of  electrons  (Lorentz), 

"  302-311  ;  electro  -  magnetic 
(Langevin),  310,  321. 

Thorium,  277,  283. 
X,  281 

Torsion  balance,  measurements 
by,  46. 

ULTRA-VIOLET  radiations,  178, 

197. 
Unit,  natural,  30;  fundamental, 

30,  37,  42  ;  of  time,  34,  36  ; 

of  volume,  36  ;  of  mass,  36  ; 


of  length,  36  ;  mechanical, 
37  ;  of  temperature,  37 ;  of 
work,  37 ;  secondary,  37  ; 
derived,  42  ;  photometric,  44. 

Universe,  life  of,  continuous,  82. 

Uranium,  270,  271,  272,  277. 
X,  281. 

VACUA,  Soddy  and  Dewar  on, 
122,  123. 

Valency,  or  atomicity,  of  an 
element  defined,  167. 

Vibrations,  luminous  arid  son- 
orous, 49,  173,  179,  180,  187, 
298  ;  transverse,  impossible 
in  fluid,  173. 

Viscosity  of  solids  and  liquids, 
128.  129,  133. 

WALLS,  semi-permeable,  147} 
148,  149. 

Water,  compressibility  of,  107  ; 
condensation  of  water-vapour 
by  ions,  242-249  (see  Ions). 

Waves  of  light,  electromagnetic, 
187  ;  experiments  of  Hertz, 
189-193  ;  his  application  of, 
to  transmission  of  signals, 
222,  223,  225, 227,  233;  appar- 
atus for  production  of,  227  ; 
action  of  electric,  on  metallic 
powder,  230  ;  luminous,  pres- 
sure of,  193  ;  stationary 
waves  and  colour-photo- 
graphy, 179,  181. 

Weight,  action  of  gravity,  31. 

Weights  and  Measures,  General 
Conference  on,  30,  33  ;  Inter- 
national Bureau  of,  23,  24, 
33,  44. 

Wireless  telegraphy,  see  Tele- 
graphy. 

X-RAYS,  see  Rays. 
Xenon,  124,  125. 


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